GOODBYE saccharine, sugar and calories. Hello, aspartame.

"It's the ideal food additive," says Dr. Richard Ronk, deputy director of the Bureau of Foods for the Food and Drug Administration. One-tenth of a calorie in the amount needed to replace one teaspoon (18 calories) of sugar. Supposedly safer than saccharine, it doesn't cause cavities.

Most important, it tastes like with saccharine. It tastes a little perfumey alone, but when it's in something like lemonade, you can't tell the difference between it and sugar.

The FDA, in a major decision, approved aspartame against the advice of a scientific panel. Although its immediate use is limited by imperfect technology, aspartame is the first product that stands a chance at cracking the sugar monopoly. Make no mistake about it, aspartame is not here merely to do battle with saccharine; aspartame is out to conquer the sugar market, to replace sugary sweetness wherever possible, slashing its calorie contribution by about 95 percent.

In the past, the FDA worried so much about artificial sweeteners -- saccharine and cyclamates -- that it categorized them as special dietary foods, with warnings that they be used only by people who for medical reasons need to restrict calories.

But aspartame, a nutritive sweetener made from two naturally occurring amino acids, the building blocks of protein, has no such strings. It can go into cereals, frostings, ice cream, chewing gum, pie fillings, powdered drinks (Ronk predicts the biggest immediate use will be in Kool-Aid) -- all of them common foods, frequently consumed by children, who may or may not need to restrict calories.

For now, aspartame won't be put into carbonated soft drinks simply because the manufacturer, G. D. Searle & Co., didn't ask for such clearance. The company plans to ask for it soon, however, and one FDA official predicts automatic approval.

If all goes smoothly, by the beginning of 1982 aspartame will be sold as a table-top sweetener in both liquid and powdered form. The FDA says it's allowing aspartame to be marketed only in two-teaspoon packets to discourage the widespread use of aspartame for cooking. But Searle's Dick McGraw says the company is combining aspartame with a bulking agent in two-pound boxes to be used just like sugar in recipes, and is putting out recipes that show how to use aspartame.

Technically, aspartame has two drawbacks that hamper its total acceptance as a sugar substitute. When left in liquid for a long time (more than six months, Searle says) or subjected to high heat, it becomes a sour-tasting chemical called diketopiperizine.That's why aspartame currently can't go into things like cake mixes and canned puddings, which are processed with high heat. And it's risky to boil or bake with it at home. But Searle is finding ways around that. For example, McGraw says the company has a brownie recipe in which aspartame is added near the end of the baking to avoid creation of the sour chemical.

Aspartame currently is much more expensive than saccharin and costs about the same as sugar per sweetening value, says the company, and "is not expected to raise the price of foods containing it." Moreover, the July issue of Food Technology magazine describes a new way to produce aspartame with DNA synthesis, which observers say could cause the cost of the sweetener to "plummet."

In a country where people are plagued by overweight, cavities and a wide spectrum of possible dangers from sugar, such as high blood pressure, heart disease and hyperactivity, it's hard to knock a product that may let us eat our cake safely. Even arch food additive foe Dr. Michael Jacobson, director of the Center for Science in the Public Interest, says: "Sure, a harmless chemical like asparatame -- if it is safe (he emphasizes he hasn't reviewed the safety literature) -- would be better than sugar or a cancer-causing artificial sweetener."

But there are reservations. Jacobson says he's afraid companies will add aspartame to junk foods to make them more attractive to kids. "How good the cereal with aspartame is," he says, "depends on the quality of the cereal." He also fears aspartame might accustom kids' tastes to prefer that everything taste sweet.

Greater cries of alarm come from Dr. John Olney, professor of psychiatry at Washington University Medical School, who since 1973, along with Washington, D.C., attorney James S. Turner, has fought FDA approval of aspartame. Olney did studies showing brain damage to rats who were given high doses of aspartame. He admits that while small amounts of aspartame might not cause similar harm to them, children are more sensitive to it than adults. More serious, Olney claims, are studies done by Hazelton Labs showing 12 brain tumors in 320 rats fed aspartame.

In fact, a three-man scientific board of inquiry established by the FDA to resolve the fight between Olney-Turner and Searle recommended in September 1980, that more definitive studies on brain tumors be done before the approval of aspartame. But new FDA Commissioner Arthur Hull Hayes Jr., swept aside that advice and gave aspartame swift approval.

"It was the right decision," says Ronk and several other FDA staffers familiar with the issue. Ronk claims the board of inquiry members were not specialists in cancer, and that an extensive examination of National Cancer Institute data showed the brain tumors were not statistically significant. Searle did other studies that showed no danger from brain tumors. Olney calls that nonsense and claims he has evidence the studies Searle did were not done properly.

It's not that Olney claims aspartame causes brain tumors; he just doesn't believe it has been proven absolutely safe. To try to force that evidence into the open, he says, within 90 days he and Turner will ask for a federal court injunction to prevent the marketing of aspartame.

If aspartame gets marketed, FDA's head of the Bureau of Foods, Dr. Sanford Miller, vows to monitor its use so Americans aren't overdosing on it to a hazardous extent. But it's likely the public would soon come to rely on it. And considering that a calorie-conscious Congress refused to let the FDA ban the potential carcinogen saccharin, the chances of getting rid of aspartame once we have it would be slim indeed.

The argument over the safety of aspartame will continue, but at least aspartame is not the proven animal carcinogen that saccharine is. And, barring an Olney-Turner success, aspartame will be on the market -- for better or worse. It will be a long time before we know whether it has any dangerous effects -- or even whether it helps make us thinner and healthier.