Amino acid concentration in the anterior preoptic area and medial basal hypothalamus was determined by HPLC in female rats: (1) at 16 (prepubertal) vs. 30 (peripubertal) days of age and (2) after N-methyl-D-aspartate (NMDA) or dizocilpine (MK-801) administration in both groups. 30-day-old rats had higher levels of aspartate (Asp; 24%), glutamate (Glu; 49%) and glycine (Gly; 44%) and lower levels of taurine (Tau; 43%) than 16-day-old rats. In 16-day-old rats, NMDA (30 mg/kg, s.c, 10 min) increased the Glut concentration (48%). This effect was prevented by MK-801 pretreatment (1 mg/kg, s.c, 1 h), which did not modify amino acid concentrations per se. In 30-day-old rats, NMDA treatment increased Glut (24%) and asp (42%) levels/lvIK-801 pretreatment abolished NMDA-induced changes and reduced Tau (26%) and Gly (30%) levels. MK-801 administration alone reduced the concentration of Glut (39%), Asp (54%), Tau (33%) and Gly (31%). It is concluded that both (1) the concentration of Asp, Glu, Gly and Tau and (2) the changes induced by NMDA receptor activation or blockade are different at 16 vs. 30 days of age. The existence of a tonic (positive) control on amino acid levels linked to the NMDA receptor which would be immature or absent at 16 days of age is suggested.