To address the question whether an increase in insulinemia and/or glycemia affects the total activity of lipoprotein lipase (LPL) in circulation, the enzyme activity was measured after periods of hyperinsulinemia (HI), hyperglycemia (HG), and combined hyperinsulinemia and hyperglycemia (HIHG) induced by euglycemic hyperglycemic clamp, hyperglycemic clamp with the infusion of somatostatin to inhibit endogenous insulin secretion, and hyperglycemic clamp, respectively. The results obtained were compared to those after saline infusion (C). Twelve healthy normolipidemic and non-obese men with normal glucose tolerance were included in the study. At the end of each clamp study, LPL activity was determined first in vivo using an intravenous fat tolerance test and then in vitro in postheparin plasma. Whereas isolated HI had no effect on LPL activity in postheparin plasma, both HG and HIHG reduced LPL activity to 60 % and 56 % of that observed after saline infusion. Similarly, the k2 rate constant determined in intravenous fat tolerance test was reduced to 95 %, 84 %, and 54 % after periods of HI, HG, and HIHG, respectively. The activity of hepatic lipase, another lipase involved in lipoprotein metabolism, was not affected by hyperinsulinemia and/or hyperglycemia. In conclusion, our data suggest that hyperglycemia per se can downregulate the total LPL activity in circulation.