The bartender in the Old Ebbitt Grill took a quiet look at his charges, then gazed wistfully down the bar at a long-abandoned Kahlua and cream. "I hate Kahlua," groaned our hero. "I mean I'm on record hating Kahlua."

A smile, and a quick, nervous swipe with a bar towel. "Right now I'd kill for one."

The Old Ebbitt's barman and a host of likeminded-revelers are taking part in, if not exactly celebrating, a Washington tradition as time-honored as State of the Union messages and Super Bowl Sunday.

It's the January Wagon - a thoroughly unorganized, voluntary pledge to abstain from strong drink after New Year's Day. Short hitters, Yuletide tipplers and those inclined to Dubonnet need not apply.

The January resolve of Washington's capacious tribe of gray-suited barroom regulars is most evident in downtown pubs and the saloon rows along M Street and on Capitol Hill. John Pelger, who has taken a turn on the wagon himself, has manned the daytime bar at the Exchange on Pennsylvania Avenue for the past five years. He babysits these January rites with patience and amusement.

"I always wish them luck," says Pelger. "It's a gallant effort I have to wait out every January."

"Some of these guys, a lot of them really, have their own system," Pelger reports. "This one guy - one of my best customers, in fact - didn't touch a beer for five whole days. God, he was happy with himself. No beer. Spent the week in here drinking vodka martinis."

The folks at the Restaurant Association of Metopolitan Washington concede that business slackens noticeably during January. An association spokesman, however, was loath to blame the slump on reform. "People just aren't as active in January," says vice-president John Cockrell. "The restaurant business in the whole area slows down this time of year."

Figures from the Distilled Spirits Council of the United States (yes. DISCUS) reflect this slowdown in the whiskey trade. January and February are traditionally the most sluggish months in both wholesale and retail markets. January sales for 1976, for example, accounted for only 7.1 percent of annual sales, as opposed to 12.6 percent in December.

"The January Wagon is the big one," claims the Veteran, a Washington lawyer with 20 years of going to lunch behind him. "Lent gets a lot of people cranked up, but January is more ecumenical."

Along with being ecumenical, the movement is up to its credit line in reasons. A portion of this spirituous somnambulsim can be attributed to the built-in emotional letdown that follow our High Holidays.

Dr. Morris Chafetz, founding director of the National Institute on Alcohol Abuse and Alcoholism, sees January abstinence as part of a larger cultural pattern. "Going on the wagon is a fairly common phenomenon this time of year," says Dr. Chafetz. "It's simply a hangover effect similar to the one in retail business.

"As a society, we're prone to excesses. The holiday season pushes us to overdo it - an excess of joy, an excess of spending, and of course, an excess of drinking.

"Moderation is not the American style," says Chafetz. "We set up systems of either/or - too much or none at all - which is why we engage in so many excesses."

Guilt, the false promise of the new year, poverty, the fear that drinking may be playing too large a part in one's life - all are perfectly good reasons to stop drinking. They are not the reasons you will hear, however, from the fellow sitting next to you swilling club soda. What he'll want to talk about is his waistline.

"I can tell you there's a lot more to this than losing weight," says the Veteran. "I went on the wagon every January for years. Did it lots of times before it took.

"Most of the people I know do it just to show themselves 'hey, there's no point quitting altogether because I can handle it. What I will do is stop drinking for a month.' The whole business is all tied up in guilt, the hairshirt, fear of the unknown.

"Weight is the easy copout," the Veteran says, "the easy thing to tell people." Barman Pelger agrees, as does Brian Knight, the man behind the bar in Nathan's. "Nobody on the wagon is going to tell you he's got a problem," says Knight. "It's always 'My doctor told me to drop 10 pounds.' or 'I put on a bunch of weight over Christmas.'"

Beyond the avoirdupois coverup, the Veteran espouses a theory more, shall we say, metaphysical, more cerebral - percipient musings from a man who knows and loves his subject.

"Over Christmas in this town," he says, "you're forced to see so many yoyos trying desperately to have a good time. You're exposed to the worst, the most excessive brand of planned hilarity on the Christmas party circuit, and if you've got a scintilla of introspection left by New Year's Day, well, you have to react. Am I like them? Did I spend last year acting like that?If I did, what am I going to do about it? Easy - I'll go on the wagon for a month."

Alas, the Veteran's conjectures offer cold comfort to members of that unhappy band sustaining themselves with grapefruit juice. Or tonic water or, as the man said, whatever gets diet soda or Virgin Marys or iced tea or, like the man said, whatever gets you through the night.

Perrier water, a naturally sparkling, moderately expensive mineral libation, has attracted a few of Washington's more impetuous abstainers. Perrier supplies the sense, the feel, the illusion of adult beverage, and local entrepreneurs get in the spirit by pricing this bubbly chimera right up there with the hard stuff. A 6 1/2 ounce dollop of Perrier can set our abstemious hearties back as much as $1.25 in some of the tonier watering holes.

The social consumption of bottled water is still in its incipient stages, but given America's obsession for devouring anything trendy, our allies in the south of France may soon be boring more holes to meet the demand. At Eagle Liquors in Georgetown, manager George Diamond reports that "a year ago we would sell a case of Perrier a month. Now it's up to 15 or 20 cases, and with a lot of advertising, we're likely to sell much more."

Our bartender in the dusky confines of the Old Ebbitt Grill is looking down the barrel of Week Four, and is leaning heavily on the bottled water. "Got a whole new system," he announces. "Perrier and soda, and plenty of it. I can't handle the taste, but I love the way it sounds."

John Pelger, meanwhile, waits patiently for Feb. 1, or St. Patrick's Day, or the promotion/demotion that kicks his regulars off the wagon.

"The regulars, you know, they come in anyway," says Pelger. "They drink coffee and order lunch and watch other people drink, so it doesn't affect me a great deal.

"Like I said, I wish 'em luck."