It has been proposed that high plasma free fatty acid (FFA) levels observed in patients with non-insulin dependent diabetes mellitus (NIDDM) contribute to the development of their insulin resistance. We examined patients with NIDDM to find whether maintaining plasma FFA levels in the fasting range with a euglycemic hyperinsulinemic clamp combined with an oral glucose load (clamp OGL) would affect insulin-mediated peripheral glucose uptake (PGU) and splanchnic glucose uptake (SGU). Nine NIDDM subjects (age, 55 +/- 3 years; duration of diabetes, 11 +/- 2 years; body mass index, 21.0 +/- 0.4 kg/m2; hemoglobin A1c, 9.0 +/- 0.3%; fasting plasma glucose, 9.4 +/- 3.0 mmol/l, means +/- SEM) were hospitalized and treated with diet, oral hypoglycemic agents or insulin for at least 2 weeks to maintain fasting plasma glucose < 8 mmol/l. All the patients were subjected to two different protocols in a random order. On one protocol, under the hyperinsulinemic condition, FFAs were maintained at the their fasting levels (1.19 +/- 0.08) by triglyceride emulsion infusion (Lipid infusion study, L), and on the other protocol, FFAs were made to fall (0.26 +/- 0.06 mmol/l) with saline instead of triglyceride emulsion infusion (Saline infusion study, S). During euglycemic (L, 5.4 +/- 0.2; S, 5.1 +/- 0.2 mmol/l) hyperinsulinemic (L, 1377 +/- 108; S, 1328 +/- 67 pmol/l) clamp, high FFA levels significantly reduced PGU (L, 26.7 +/- 3.6; S, 32.1 +/- 3.4 mumol.kg-1.min-1, P < 0.05) and SGU (L, 12.1 +/- 4.2; S, 27.5 +/- 5.6%, P < 0.05). In conclusion, high FFA levels in patients with NIDDM impaired insulin-mediated glucose uptake in the splanchnic as well as peripheral tissues.