Abstract In 14 obese male subjects (whose plasma triglyceride (TG) levels lay between 72 and 491 mg/dl), total plasma cholesterol, plasma TG, “hepatic” TG lipase (HTGL) and (“extrahepatic”) lipoprotein lipase (LPL) activity in post-heparin plasma, and the k 2 of the intravenous fat tolerance test (IVFTT) were measured when on a normal isocaloric diet and after one and two weeks of total starvation. A pronounced and significant decrease of plasma TG and a less pronounced but (after two weeks) also significant decrease of plasma cholesterol concentration during starvation were found. LPL activity was not significantly influenced by starvation for two weeks. There was a significant decrease of HTGL activity after one and two weeks of starvation. On a normal diet, a negative correlation existed between TG and k 2, but no relationship could be demonstrated between the lipase activities and k 2 or between the lipase activities and plasma TG. After one week and after two weeks of starvation, inverse correlations between k2 and TG, as well as between LPL activity and plasma TG, and positive correlations between LPL activity and k 2 were found. The results of this study are compatible with the following hypothesis concerning the regulation of plasma TG concentration. Both on a normal diet and during prolonged starvation, a reduced fractional removal rate of plasma TG is responsible for an increase in plasma TG concentration. Independent of the nutritional state, k2 seems to be a reliable estimate for the fractional removal rate of endogenous plasma TG. In the fed state, the extent of hypertriglyceridaemia is also influenced by the influx rate of plasma TG.