To study the effect of changes in plasma non-esterified fatty acid concentration on suppression of hepatic glucose production by insulin eight Type 2 (non-insulin-dependent) diabetic patients participated in three euglycaemic, hyperinsulinaemic (108pmol.m2-1.min-1) clamp studies combined with indirect calorimetry and infusion of [3-3H]-glucose and [1-14C]palmitate; (1) a control experiment with infusion of NaCl 154 mmol/l, (2) heparin was infused together with insulin, and (3) an antilipolytic agent, Acipimox, was administered at the beginning of the experiment. Six healthy volunteers participated in the control experiment. Plasma non-esterified fatty acid concentrations during the insulin clamp were in diabetic patients: (1) 151 +/- 36 mumol/l, (2) 949 +/- 178 mumol/l, and (3) 65 +/- 9 mumol/l; in healthy control subjects 93 +/- 13 mumol/l. Non-esterified fatty acid transport rate, oxidation and non-oxidative metabolism were significantly higher during the heparin than during the Acipimox experiment (p less than 0.001). Suppression of hepatic glucose production by insulin was impaired in the diabetic compared to control subjects (255 +/- 42 vs 51 +/- 29 mumol/min, p less than 0.01). Infusion of heparin did not affect the suppression of hepatic glucose production by insulin (231 +/- 49 mumol/min), whereas Acipimox significantly enhanced the suppression (21 +/- 53 mumol/min, p less than 0.001 vs 154 mmol/l NaCl experiment). We conclude that insulin-mediated suppression of hepatic glucose production is not affected by increased non-esterified fatty acid availability. In contrast, decreased non-esterified fatty acid availability enhances the suppression of hepatic glucose production by insulin.