Increased circulating levels of nonesterified free fatty acids (NEFA) have been observed in such hyperinsulinemic states as obesity, impaired glucose tolerance, diabetes, and dyslipidemia where they have been causally linked to the development of insulin resistance and hyperinsulinemia. The concentration of NEFA in plasma is believed to have direct modifying effects on insulin secretion and clearance. It remains controversial whether acute increases in NEFA potentiate insulin secretion in human subjects. We studied the effect of an acute elevation of NEFA during lipid-heparin infusion compared to a glycerol-only control on glucose-stimulated insulin secretion and clearance during a 120-min hyperglycemic (10 mM) clamp in 7 healthy normoglucose-tolerant volunteers. The metabolic clearance rate of C-peptide (MCR(CP)) was measured in each subject during the study by simultaneous infusion of C-peptide. Insulin secretion rate (ISR) was calculated from deconvolution of C-peptide data after correction for the rate of C-peptide infusion. Clearance rate of insulin (MCR(INS)) was calculated based upon endogenous ISR. Plasma glucose (mg/dL): basal (90-115 min) 90.2 +/- 2.8 vs. 90.2 +/- 2.3; clamp (150-240 min) 180.5 +/- 2.8 vs. 180.9 +/- 1.3. Plasma insulin (pmol/L): prebasal (fasting) 29.6 +/- 10.0 vs. 29.8 +/- 10.6; basal (90-115 min) 30.1 +/- 9.2 vs. 34.5 +/- 12.1; second phase clamp (210-240 min) 127.6 +/- 18.2 vs. 182.5 +/- 17.3*. Plasma NEFA (mM): prebasal 0.47 +/- 0.08 vs. 0.52 +/- 0.09; basal 0.35 +/- 0.05 vs. 0.98 +/- 0.02*; clamp (122-240 min) 0.06 +/- 0.02 vs. 0.77 +/- 0.06*. ISR (pmol/min): prebasal 72.7 +/- 7.5 vs. 72.0 +/- 7.9; second phase clamp (210-240 min) 268.5 +/- 27.2 vs. 200.2 +/- 23.7. MCR(INS) (mL/min): prebasal 3393 +/- 488 vs. 3370 +/- 511; clamp 2284 +/- 505 vs. 1214 +/- 153* (*p < 0.05 glycerol vs. intralipid/heparin). This study demonstrates that acute NEFA elevation causes hyperinsulinemia due to a significant decrease in systemic insulin clearance without increasing rates of insulin secretion.