In order to study the metabolic effect of dietary sucrose or cornstarch on plasma total triglyceride lipase (T-TGL), hepatic triglyceride lipase (H-TGL) and monoglyceride hydrolase (MGH) activities, young male Zucker fatty and lean rats were pair-fed or fed ad libitum for 4 weeks with diets containing 68% carbohydrate as either sucrose or cornstarch. Our results show that ad libitum feeding of fatty rats with both diets produced an intensification of plasma hypertriglyceridemia which was accompanied by high levels of all plasma postheparin lipolytic activities. These diets did not affect the enzyme activities of lean littermates. Pair-feeding the fatties with sucrose-rich diet produced an increase in T-TGL, H-TGL and MGH. If cornstarch was the carbohydrate included in the diet no difference in postheparin lipolytic activities was found between phenotypes.