The deposition of edidymal and perirenal fat, serum insulin levels, and insulin sensitivity of epididymal fat, expressed as the insulin-stimulated production of CO2 from glucose, were determined in Wistar rats fed diets containing either 54% starch or sucrose ad libitum or pair-fed in meals. Regardless of the pattern of feeding, sucrose-fed rats deposited more adipose tissue per 100 g body weight and exhibited less insulin sensitivity than did starch-fed rats. Significant differences in adipose tissue weights were not always accompanied by significant differences in body weights. Meal-fed rats deposited less adipose tissue and showed a greater insulin sensitivity than did ad libitum rats fed the same carbohydrate. However, when changes in feeding pattern negated the difference in adipose weights there was no difference in the insulin sensitivity of the meal-fed and ad libitum-fed rats. Rats consuming the sucrose diet generally exhibited significantly higher fasting serum insulin levels than did rats consuming the starch diet. The serum insulin values tended to be higher in the ad libitum-fpididymal tissue from the meal-fed and starch-fed rats tended to be greater than that of the sucrose-fed or ad libitum-fed rats, respectively, suggesting differences in adipocyte composition. Since obesity, insulin insensitivity, and hyperinsulinism are associated with an impairment of glucose tolerance, the observed metabolic effects of dietary sucrose are considered to be undesirable.