Aminooxyacetic acid (AOAA), an inhibitor of gamma-aminobutyric transaminase, stimulates the in vitro GABA release by medial and anterior preoptic hypothalamic areas in prepubertal female rats (6, 15 and 30 days of age). This increase of GABA release at 15 days of age, was accompanied by a significant increase (P<0.01) in the hypothalamic release of glutamate (GLU) and aspartate (ASP), the excitatory amino acids involved in N-methyl-D-aspartate neurotransmission and a decrease in the release of these excitatory amino acids at 6 and 30 days of age (P<0.01). The increase in the hypothalamic release of GLU and ASP at 15 days of age was accompanied by a significant increase of the plasmatic LH and FSH concentration, while the hypothalamic decrease of excitatory amino acids release induced by AOAA also decreased LH and FSH plasmatic levels at 6 and 30 days of age. In summary, the present results show that in female rats there are differences in the effect of GABAergic system the hypothalamic release of GLU and ASP and on gonadotrophin secretion at different ages of prepubertal period, i.e. an inhibitory effect at 6 and 30 days of age and a stimulatory one at 15 days of age. It is proposed that the different effects of GABA on gonadotrophin secretion in prepubertal rats previously described are connected with ontogenic changes in the interrelationships between GABAergic and NMDA neurotransmission systems during sexual maturation of the hypothalamus in female rats. It is probable that these ontogenic modifications are connected with the maturation of interneuronal connection and/or new receptors activity.