To study the relationship of endogenous GHRH to the changes seen in serum GH concentration in neonatal primates, we administered antiserum to GHRH (GHRH-Ab) or GHRH antagonist, (N-Acetyl-Tyr¹, D-Arg²)GHRH-(1-29)-NH₂, to unanesthetized, fasted rhesus monkeys, 1–31 days of age, via an acutely placed venous catheter. The administration of GHRH-Ab resulted in a significant decrease in serum GH concentration at 1–2 and 7–9 days, but not at 28–31 days of age. The administration of GHRH antagonist resulted in a significant decrease in serum GH concentrations at 1–2 days, but not at 7 or 14 days of age. These results imply that basal serum GH is more dependent on endogenous GHRH at 1–2 days than after 7–28 days of age. This suggests developmental changes in the hypothalamic regulation of GH secretion with advancing age in the neonatal period.