Coca-Cola Co., following the lead of competitors PepsiCo Inc. and Seven-Up Co., said yesterday it will replace the saccharin in Diet Coke with the sweetener aspartame.

Two-liter bottles of the reformulated soft drink will begin appearing within days in many sections of the country, and Coke hopes to have cans and bottles of aspartame-sweetened Diet Coke on all store shelves around the country shortly after the first of the year, about the same time as aspartame-flavored Diet Pepsi and Diet 7-Up.

The company said it had not decided yet whether to switch its other diet drinks -- Tab, Diet Sprite and Caffeine-Free Diet Coke -- to aspartame sweetening. It also said that the soda-fountain version of Diet Coke would be unaffected by the change for now because of uncertainty about whether aspartame retains its sweetness over long periods.

Coke, like the other soft-drink makers, has been using a mix of about 80 percent saccharin to 20 percent aspartame in its diet drinks for the past 15 months. The companies had held off going entirely to aspartame because of concerns about the ingredient's supplies, high costs and possible health problems.

The conversion of Diet Coke, the nation's third-most-popular soft drink, to aspartame is another breakthrough for G. D. Searle & Co., which developed aspartame and sells it under the trade names NutraSweet and Equal, and, conversely, another setback for saccharin. Sherwin-Williams Co., the major domestic saccharin producer, recently put the division that makes the sweetener up for sale.