Given the potential use of a low-calorie sweetener during weight reduction, a toxicity study of chronic aspartame ingestion was conducted. Particular attention was given to possible long-term effects of aspartame on the fuel hormonal alterations characteristically caused by weight reduction. As a group mean age was 19.3 yr, body weight was 164.6 lb, and mean height was 65.4 in. Subjects were an average of 33% in excess of ideal body weight. The aspartame dose was 2.7 g/day and was compared on a double-blind randomized basis with a lactose placebo. Both materials were given in gelatin capsules. An average of 6.9 +/- 1.5 lb was lost by the aspartame group during the 13-wk study on a calculated 1,000-calorie diet. The placebo group lost 4.5 +/- 1.2 lb (no significant difference between the two groups). After an overnight fast, reductions in glucose and immunoreactive insulin were seen in both groups, while rising trends in immunoreactive glucagon were observed. These changes are all characteristic of calorie restriction. In no instance was there a detectable effect of the ingested aspartame. No meaningful effect of weight reduction or aspartame was seen on plasma triglyceride and cholesterol, nor on any other parameter of hematologic, hepatic, or renal function that was measured. Similarly, side effects were equally distributed between asparatame and placebo.