Studies were undertaken to examine triglyceride turnover in obese humans on isocaloric balanced diets and during prolonged (3-5 wk) fasting. The data were related to plasma concentrations of insulin (IRI), glucagon (IRG), and free fatty acids (FFA) and to blood ketone concentrations. The triglyceride turnover rates were also related to the plasma triglyceride concentration. This relationship was the same in the obese on isocaloric balanced diets as that we have previously observed in lean humans on similar diets. The relationship between triglyceride turnover and concentration changed during prolonged fasting in a way that suggested that triglyceride removal was impaired. This viewpoint is consistent with the known effects of fasting on adipose tissue lipoprotein lipase activity. In another group of fasted obese, refed with a hypocaloric diet, the relationship returned toward normal. In addition to the impaired triglyceride removal, prolonged fasting resulted in a decrease in triglyceride production. This decrease occurred despite an increase in plasma FFA. After 3-5 wk of fasting the IRI was about 50% of the initial value, while the IRG was the same as the initial value. While triglyceride production fell during fasting, the blood ketone concentration rose. Others have seen similar changes in ketones and triglycerides in livers perfused with medium in which the ratio of insulin to glucagon fell. The rate of triglyceride production was not related to body weight. However, regardless of nutritional state, it was positively related to the basal plasma insulin levels. These data indicate that, in man as in animal preparations, insulin may regulate hepatic triglyceride production.