To test the hypothesis that acetylcholine-induced relaxation of the renal artery decreases with postnatal age, we measured parameters of renal hemodynamics before and for 35 s after aortic suprarenal injection of acetylcholine in conscious, chronically instrumented lambs aged approximately 1 wk (n = 5) and approximately 6 wk (n = 5). Acetylcholine was administered in one of five doses ranging from 0 to 10 mg/kg body wt; doses were administered randomly, in the same volume. There were significant age- and dose-dependent changes in renal vascular resistance after acetylcholine administration, such that the response was greater in 1-wk-old lambs. After the highest dose tested, renal vascular resistance decreased by 13.6 +/- 7.3 (SD) mmHg. ml(-1). min. g kidney wt in 1-wk-old lambs and by 9.1 +/- 3.2 mmHg. ml(-1). min. g kidney wt in 6-wk-old lambs at 35 s. We also observed a transient renal vasoconstriction before the renal vasodilatation in 6-wk-old lambs but not in 1-wk-old animals. These data provide the first age- and dose-dependent effects of exogenous administration of acetylcholine on renal hemodynamics during maturation in conscious animals.