To determine whether differential response to growth hormone-releasing hormone (GHRH) could cause the developmental changes seen in growth hormone (GH) secretion, we administered 10 µg/kg GHRH (1-44 NH2) to a group of four unanesthetized, fasted, rhesus monkeys via acutely placed venous catheters at 1, 7, 14, and 28 d postnatal age. Serum GH was assayed by hGH RIA in sera collected at -60, -30, 0, 15, 30, 45, 60, and 90 min relative to the GHRH bolus. Serum cortisol was measured by ELISA in the 0-, 30-, and 60-min samples. Differences between age groups were analyzed by repeated measures analysis of variance and pairedttests. Mean basal GH levels were higher at 1 d (9.4 ± 1.2 µg/L, mean ± SEM) than at 7 (5.5 ± 0.4), 14 (5.6 ± 0.5), and 28 d (5.3 ± 0.5) of age. There were no other significant differences in mean basal δGH values between the age groups. Mean post-GHRH GH concentrations decreased significantly with each age after 1 d (22.6 ± 1.6): 7 d (16.4 ± 1.3); 14 d (11.3 ± 1.0); and 28 d (7.9 ± 0.9). Similarly, mean δ GH values decreased with each increase in age from 1 d (15.0 ± 1.9): 7 d (10.9 ± 1.6); 14 d (5.9 ± 1.1); and 28 d (2.7 ± 0.8). Serum cortisol was not correlated with serum GH at any age. Our study demonstrates decreasing basal GH concentration and GH responses to GHRH with advancing age from 1 to 28 d in the rhesus monkey. The spontaneous decline in GH concentration observed during the first few days of life may reflect a decrease in pituitary gland response to GHRH stimulation.