In male rats, a GABA(A) receptor antagonist, bicuculline methiodide or an agonist, muscimol, was infused intravenously for 30 min into the male rat before (postnatal days 16-17 and days 30-31) and after (over day 45) the sexual maturation. Neither bicuculline nor muscimol infusion significantly altered serum LH concentrations at days 16-17. At the days 30-31 and after day 45, bicuculline infusion significantly increased and muscimol infusion significantly decreased serum LH. In conclusion, in male rats, the functional GABA(A) receptor system to inhibit the release of LH seems to be established before the sexual maturation, between 18 and 29 days of age.