The increased triglyceride and decreased HDL commonly associated with diabetes are of concern because of their possible role in the etiology of the greatly increased cardiovascular disease in this disorder. Moreover, it has not been established how therapy of diabetes, especially using oral hypoglycemics, influences lipoprotein metabolism and lipase activities. This study compares VLDL metabolism, FFA metabolism and lipoprotein lipase activities in type II diabetes before and after therapy with the oral hypoglycemic agent tolinase. The results showed that improvement of glycemic control is followed by significant falls in VLDL-TG and LDL cholesterol and reversal of abnormalities of HDL subfractions. VLDL composition was abnormal in diabetics and reversed after therapy. Data on VLDL metabolism in diabetics revealed a definite clearance defect in VLDL metabolism mediated by decreases in lipoprotein lipase. The data also suggested that diabetes influences VLDL-TG production independent of that of apoB, possibly through elevations of free fatty acids.