The delayed effect of growth hormone on renal function in humans. Growth hormone is reported to increase renal plasma flow (RPF) and glomerular filtration rate (GFR) in some but not all studies. The discrepant results could be due to a delay in the effects of growth hormone on renal function. We therefore examined whether a growth hormone injection does increase RPF and GFR, whether this increase is delayed, and whether elevation in RPF and GFR is associated with increased plasma levels of insulin-like growth factor I (IGF-I). Seven normal adults received a single intramuscular injection of growth hormone, 0.15 mg/kg, and serial PAH and inulin clearances were then monitored for three consecutive days. Plasma growth hormone levels peaked an average of 2.25 hours after injection, at 128 ± 12 SEM ng/ml, and then began to decrease; on the second day values were only slightly elevated and on the third day they were not different from baseline. Plasma IGF-I, analyzed by direct radioimmunoassay, did not change on the first day during 5.5 hours of measurements after injection. By the second day, plasma IGF-I was elevated to over twice baseline levels (P < 0.05) and remained elevated on the third day (P < 0.05). RPF and GFR did not change from baseline (546 ± 19 and 100 ± 3 ml/min/1.73 m², respectively) during the 5.5 hours after injection on the first day. By the second day, RPF had risen to 715 ± 21, P < 0.05, and GFR to 130 ± 5 ml/min/1.73 m², P < 0.05, and mean values remained elevated on the third day. The grand mean for total renal vascular resistance was decreased on the second and third days. Thus, after a growth hormone injection there was a rise in RPF and GFR which was delayed, was observed after plasma growth hormone levels had fallen to baseline, and which occurred in association with the increased plasma IGF-I levels. These findings suggest that growth hormone increases RPF and GFR indirectly through one or more mediators, one of which could be IGF-I.