The present results indicate that during sexual maturation the APOA-MBH from rats of 30 days of age released significantly higher quantities of GnRH than the tissue from 16-day-old rats (P < 0.01). The addition of NMDA, an agonist of the excitatory amino acids system (EAAs), to the medium after 30 min of incubation significantly increased (P < 0.01) the GnRH release in normal rats of both ages and this increase was significantly (P < 0.01) higher in 30-day-old rats (to 661%) than in rats of 16 days of age (to 273%). The administration of estrogen-progesterone (EP) to rats of 16 days of age did not modify the GnRH release response to NMDA. On the contrary, at 30 days of age EP administration significantly potentiated the GnRH release response to NMDA since while in the control group NMDA increased the GnRH release to 630%, in the EP-pretreated group this was to around 4700% (P < 0.01). EP pretreatment of prepubertal rats decreases the hypothalamic release of aspartate and glutamate, the excitatory amino acids involved in NMDA neurotransmission and glycine but increases EAAs release in peripubertal rats. On the basis of these results it is proposed that the increase in EAAs release by the hypothalamus is directly connected with the onset of puberty and that the maturation of the positive feedback effect of ovarian hormones on gonadotropin secretion is related to the maturation of the capacity of EP to increase hypothalamic EAAs. Before this maturational event EP inhibits EAAs release as well as gonadotropin release (prepubertal rats). NMDA receptor stimulation leads to a positive mechanism which increases the release of Asp and Glu from APOA-MBH both in prepubertal and peripubertal rats, but EP potentiates this mechanism only in peripubertal rats. This could be an additional neuroendocrine mechanism involved in the increase of gonadotropin during sexual maturation which induces the onset of puberty and the preovulatory discharge of these pituitary hormones.