The effect of activation of central γ-aminobutyric acid β (GABAB) receptors on pulsatile luteinizing hormone (LH) secretion was examined in unanesthetized ovariectomized rats. Intraventricular injections of 2 or 10 µg baclofen, a selective GABAB receptor agonist, usually arrested the existing pulsatile LH secretion for a short period, then LH pulses resumed with reduced frequencies and increased amplitudes. The increase in the pulse amplitude was not necessarily accompanied by a reduction in the pulse frequency, but was observed even after nonelongated interpulse intervals in animals receiving the GABA agonist. Mean LH levels showed complex bi- and triphasic fluctuations after the central administration of high and low doses of baclofen, respectively. It was concluded that central GABAB receptors are involved in the modulatory mechanism affecting the LH pulse frequency and amplitude in the ovariectomized r