Depolarization-induced release of neurotransmitters from nerve endings is triggered by the rapid entry of Ca through voltage-sensitive channels. Low (1 to 10 nmol/L) concentrations of L-triiodothyronine (T3), but not D-T3, reverse T3 (rT3) or L-thyroxine (T4), enhance fast-phase depolarization-induced 45Ca uptake by rat brain synaptosomes. In the present study the effects of thyroid hormones on the release of 3H-gamma-aminobutyric acid (3H-GABA) by cerebral cortical synaptosomes from adult male Sprague-Dawley rats were determined by a superfusion technique. It was found that low (1 to 10 nmol/L) concentrations of T3, but not D-T3, rT3, or T4, significantly enhanced Ca-dependent 3H-GABA release by synaptosomes exposed to a high K depolarization buffer for 5 sec. Low concentrations of T3 did not enhance basal 3H-GABA release or release produced by exposure to depolarization buffer for 30 seconds. Much higher concentrations (10 mumol/L) of both T3 and T4, but not rT3, D-T3, or D-T4, stimulated 3H-GABA release produced by exposure to depolarization buffer for 30 seconds.