N-methyl aspartic acid (NMA) was without effect on the pituitary growth hormone (GH) secretion of adult and neonatal rats in vitro. Administration of NMA resulted in a rapid rise of plasma GH levels in intact but not in arcuate-nucleus-lesioned adult rats, indicating that NMA stimulated GH-releasing hormone (GRH) secretion. In 2-day-old rats, both NMA and gamma-aminobutyric acid (GABA) elevated plasma GH levels in a dose-related fashion; GRH administration was without effect. The elevation of plasma GH levels after NMA injection was reduced by administering an antibody to GRH. These results indicate that GH secretion is partly regulated by endogenous GRH in the newborn rat but that the elevation of plasma GH levels after GABA is not mediated by GRH. The high plasma GH levels seen in the newborn rat may result from the independent action of GABA and GRH but the effect of other factors cannot be excluded either.