The metabolic and cardiovascular effects of recombinant human IGF-I were compared to insulin in six normal subjects. Subjects were studied twice and intravenously received an infusion of [6,6-2H2]glucose (0-480 min) and in random order either IGF-I 20 micrograms kg-1 h-1 (43.7 pmol kg-1 min-1 or insulin 0.5 mU kg-1 min-1 (3.4 pmol kg-1 min-1) with an euglycaemic clamp. One subject was withdrawn following a serious adverse event. During the IGF-I infusion glucose appearance rate (Ra) decreased from 1.79 +/- 0.13 at baseline (150-180 min) to 0.35 +/- 0.26 mg kg-1 min-1 (P < 0.01) at 360 min, and glucose utilization rate (Rd) increased from 1.79 +/- 0.28 to 4.17 +/- 0.84 mg kg-1 min-1 (P < 0.01). There was no change in free fatty acids (FFA) and an increase (percentage change from pre-infusion mean) in cardiac output +l37.3% +/- 9% (P < 0.01), heart rate +13% +/- 2% (P < 0.01) and stroke volume +21% +/- 7% (P < 0.05). During the insulin infusion glucose Ra decreased from 1.89 +/- 0.13 to 0.34 +/- 0.33 mg kg-1 min-1 (P < 0.01) and FFA from 0.546 mmol l-1 to 0.198 mmol l-1 (P < 0.01), glucose Rd increased from 1.89 +/- 0.18 to 5.41 +/- 1.47 mg kg-1 min-1 (P < 0.01) and there were no significant changes in the cardiovascular variables.