The nose was like an armful of herbs and dried grasses, followed by a whisper of smoke on the palate. In short, this whiskey was a bit like a noble old Scotch from Speyside, except that a splash of water brought out a bright note of anise.

And except for the fact that it wasn't a Scotch at all. It was a 21-year-old bourbon. Bourbon is usually the most extroverted of spirits, but this long-aged example &

Black Maple Hill 21 years, cask No. 5 &

was a surprise: complex and reserved, like a great scholar in his study. Like the scholar, it was worth studying.

But when we called the retailer to double-check the price, we had another surprise. In the few weeks between the time we'd bought it and our tasting, both Los Angeles stores that had been stocking it had run out, and they weren't expecting to get any more. This turned out to have been one of the last bottles.

What had made this bourbon so extraordinary? It started out, like countless other bourbons, as a mash of fermented corn, rye and barley. After being distilled, though, it spent 21 years in charred oak barrels before being bottled. That's more than five times as long as most bourbons.

It's a rarity, and long-aged bourbons like this one are, alas, becoming even more rare. Now is the time to try them &

most are going to disappear from the shelves soon, and their like won't be widely available again for a couple of years.

Bourbon gets its vanilla and caramel sweetness from the barrel wood, and the longer it spends in barrel, the more of those flavors it picks up. But as it ages, alcohol and water also evaporate through the barrel staves, about 2.5 percent per year. Walk into a bourbon rack house, and you can be overwhelmed by this heady aroma. It's called the angels' share &

but what the angels take away, they repay the whiskey with delicacy and complexity.

Cognac and Scotch also give the angels their share as they age. In fact, after 20 years or more in barrel, bourbon, Scotch and Cognac come to share a family resemblance. All become subtle, fascinating and infinitely civilized.

Who knew bourbon could age like this? For a long time, there simply was no really long-aged bourbon. Wild Turkey's eight-year was generally the oldest around.

We might never have discovered old bourbon if the bottom hadn't fallen out of the market for a couple of decades. Americans went crazy for vodka and tequila in the '70s and '80s, and bourbon sales fell month after month. To a lot of Americans, bourbon had become just something you heard about in country-western songs.

This meant bourbon distillers had unsold barrels of whiskey on their hands. Marketing companies emerged to buy them up and bottle them under their own labels. These companies came to serve the same function as "negociants" in the Burgundian wine market &

or, more exactly, as the independent bottlers of Scotch. Because of their efforts, more and more of us eventually became aware of old bourbons, leading to a small explosion after 2000.

But darn. Now you see it, now you don't. Just when we've developed a taste for these rare old bourbons, the supply is about to dry up.

The oldest of the marketers is appropriately named Old Rip Van Winkle. In the '70s, the company started selling a 10-year-old bourbon made for it by a Louisville, Ky., distiller, followed by a 12-year in the '80s. "It was half accident how it happened," says marketing manager Preston Van Winkle. "My father, Julian Van Winkle III, had some older barrels and enjoyed older bourbon himself, so he decided to release them."

Back then, the customers for these American treasures were still mostly abroad. Van Winkle's biggest market was Japan. The same was true for Kentucky Bourbon Distillers' ultra-aged Vintage Bourbon line (17-, 21- and 23-year versions are currently available), which found customers mostly in France and Germany.

Meanwhile, Americans discovered single-malt Scotch, and Jim Beam reminded the public that bourbon too can be a premium spirit with its Small Batch Collection. Interest in old bourbons slowly ticked upward in the '90s, and new companies came on line: CVI Brands started selling old bourbons under the Black Maple Hill label, and Spirits Imports introduced Classic Cask. 2005, you could find 20 bourbons older than 20 years.

Most companies that market long-aged bourbon, like some independent bottlers of Scotch, are cagey about where they find their old barrels &

they don't even reveal the name of the distiller. When they do find barrels for sale, the principle of selection is simply tasting. "It has to meet your quality standards," CVI President Paul Joseph says. "There's some old bourbon out there you wouldn't want to drink. If it's aged in hot weather, the extract from the barrel can be too much."

The exception is Preiss Imports, which freely discloses where it got its A.H. Hirsch bourbon. It was distilled at the historic Michter's distillery in 1974 and aged in barrel for 16 years. Then it was stored in stainless steel tanks (in which no aging takes place) until being bottled.

While Americans have been discovering premium aged bourbon, more foreigners have been doing the same. Japan and Europe are no longer bourbon's only markets abroad &

India, Australia and China now clamor for it. Put this all together, and you get the current shortage.

"There was never a market for older bourbons before 2000 or so," says Alan Shayne of Spirits Imports. "(Then) all those old stocks of bourbon lying around that they didn't know what to do with got bottled.

"The distilleries have noticed, though. There's going to be a shortage for a couple of years, then the supply of older bourbons and ryes will ease up."

The distilleries have definitely noticed. Older versions of familiar brands such as Old Fitzgerald, Eagle Rare and Elijah Craig have been coming on the market. And, now that bourbon is a fashionable spirit again, the company that owns Wild Turkey is spending $30 million to double the distillery's capacity, and others are following suit.

The distilleries will set aside some of that increased output for long aging, though cautiously. Aged whiskey is hostage to the fashions of 10 or 20 years in the future, so the industry instinctively hedges its bets.

But all that's down the road. For the near future, everybody expects at least a two-year shortage of old bourbon.

So if you have a taste for the old stuff, now is the time to buy. And if you flinch at the prices, here's a cheerful fact you can bear in mind: Unlike wine, whiskey doesn't change once it's bottled, so it won't spoil. It keeps indefinitely &

if you have the patience.