Dr. Brown's Cel-Ray Tonic is coming out of the closet. Not just a soft drink, but a way of life, Dr. Brown's is about to be treated to its first advertising campaign in 109 years. "Today the delicatessens," its promoters are saying, "tomorrow the world!"

For New York City in general and its Jewish community in particular, Dr. Brown's revered and defended with a fierceness that makes Dr. Pepper fans look like dilettantes.

Why? Because this beverage not only quenches your thirst with its inexpressibly exotic celery taste but serves as a talisman and a cultural rite as well, a sign of goodness and correct thinking that even Chairman Mao would have appreciated.

Say you walk into a delicatessen in New York and the counterman gives you the eye. Maybe you look too scruffy for the neighborhood, or too elegant, but whatever it is, suddenly your chances of getting that corned beef nice and lean the way you want it are slim and none, and slim just went home.

But tell the counterman you're drinking Dr. Brown's Cel-Ray Tonic and suddenly the clouds part and it's all smiles. Ordering Dr. Brown's is like using just the right fork at Lutece: Everyone knows at once you're okay.

"People are total fanatics about this stuff, that's all they want," says Harry Gold, a vice-president of the American Beverage Corporation, the folks who make Dr. Brown's and, says Gold, "feel like a bunch of fools because we've never been able to figure out who Dr. Brown is or even if there was one."

Gold is also happy to admit that Dr. Brown's Cel-Ray isn't one of those pushy, ingratiating sodas that insists on being loved at first gulp.

"How can I describe it," he says, more in awe than anything else. "It's very, very delicately flavored, and you have to develop and acquire a taste for it. There's no doubt in my mind about that and we're not ashamed to admit it. But once you do, we have you as a customer for life."

And the fact is that in the last 10 or 20 years, as New Yorkers have tentatively moved out into the Great Heartland, Dr. Brown's Cel-Ray has moved too, to some 20 markets in fact.

"With them they take their tradition, with the tradition comes the deli, and with the deli comes Dr. Brown's," explains Alice O'Leary, the Dr. Brown's account executive at the New York ad firm of Altschiller, Morgan, Reitzfeld & Jackson. "People have a very strong emotional feeling about this soda."

In the Washington area, these strong feelings get expressed at places like Katz's in Silver Spring, where one employee said, "It's not a Coca Cola but people are buying it, what else can I tell you? Why do they like it? You know how it is, a doctor in the house."

For years Dr. Brown, which also makes the classic Cream Soda and the equally classic Black Cherry at its plant across from La Guardia Airport, has been content with these kinds of sales. But now, partly because of a change in management, partly because of population shifts, a grab is being made for the main chance.

"The time may be ripe for Dr. Brown's," says Harry Hold solemnly. "You know it's such a natural and wholesome drink, its base is made from an extract of real celery seeds. And this is going to sound ludicrous, but there are only two executives in the entire company who can prepare the final formula."

With assets like these, says ad person O'Leary, it was only natural for Dr. Brown's to want to "roll out the product in a bigger way. We don't want the deli owner to feel deserted, but this is a world of reality, and to make this a profitable on-going venture with the people we're chasing, the ethnics as well as the every-day, non-ethnic consumer, we have to go to supermarkets."

So O'Leary and her agency, armed with an initial budget of half-a-million dollars and an initial 10-city target area - Boston, Chicago, Buffalo, Rochester, Syracuse, Philadelphia, Houston, Dallas, Los Angeles and of course New York - have redesigned the Dr. Brown's label and come up with nifty posters reading "Imported from the old neighborhood," "We've missed you, too," and "For prompt, temporary relief of the minor pain of nostalgia."

The only compromise Dr. Brown's had to make in its national push, and it does pain Gold a bit to discuss it, was putting the words "Celery Soda" on the main label, which meant the classic "Cel-Ray Tonic" gets relegated to an auxiliary spot.

"We may reverse that very, very soon," he says with a sign, "But when we begin to market it outside of New York, we wanted to make sure people knew what they were buying, so we bit the bullet and put 'Celery Soda' on the label."

And as far as any problems with ethnic crossovers, Gold professes to know no fear. "You don't have to be Jewish to drink Dr. Brown's," "We'll give you a special dispensation."