Serum transaminase levels were measured as part of a study on the physiologic control of food intake. Twenty-one men, 15 nonobese and six obese, were housed on a metabolic ward for 30 days where they received ad libitum a baseline diet of conventional foods containing 25 to 30 percent of total calories as sucrose for 18 days and a calorically diluted diet containing less than 10 percent sucrose for 12 days. Serum glutamic pyruvic transaminase (SGPT) and serum glutamic oxaloacetic transaminase (SGOT) levels rose significantly when subjects consumed the baseline diet and returned to their original levels on the calorically diluted aspartame-sweetened diet. Markedly abnormal transaminase levels developed in two subjects on the baseline diet and they had to be discharged from the study. Correlations between various components of the diet and enzyme changes suggested that both surplus calories and a high sucrose intake played a role in the elevation of enzyme levels. Serum triglyceride levels also showed a significant reduction when the subjects were switched from the baseline to the calorically diluted diet. In a second study designed to test systematically the role of sucrose on SGPT and SGOT levels and on serum triglyceride levels, six nonobese subjects received a carefully controlled liquid diet, relatively high in linoleic acid content, containing 50 percent of total calories as either sucrose or maltose. In comparison with the first study, sucrose had a smaller but still significant effect on the levels of both enzymes, but there was no significant effect on triglyceride levels.