This study examined the possible involvement of dopamine (DA) and/or serotonin (5-HT) in regulating basal pulsatile LH release, and in mediating perisuprachiasmatic (peri-SCN)-induced suppression of pulsatile LH secretion in ovariectomized rats. Unanesthetized animals were bled continuously through jugular vein cannulae for a 1½-h prestimulation period, 1½ h of stimulation, and up to 3/4 h after stimulation. Control rats were untreated or given d-butaclamol, the inactive stereoisomer of d-butaclamol. Other groups were pretreated either with pimozide or d-butaclamol to block DA receptors, parachlorophenylalanine (PCPA) or metergoline to inhibit 5-HT synthesis or block 5-HT receptors, respectively, or PCPA + pimozide to affect both 5-HT and DA systems. Prior to stimulation, regardless of whether DA and/or 5-HT systems were interfered with, no significant differences existed in mean blood LH levels, LH interpulse intervals, or the magnitude or rate of increase in blood LH levels for individual LH episodes. This suggests that DA and/or 5-HT may not be essential parts of the neurotransmitter systems regulating basal pulsatile LH secretion. During peri-SCN stimulation significant decreases in mean blood LH levels occurred in all groups. This was due entirely to a lengthening in the interval between LH pulses, since no group showed a decrease in the magnitude or rate of increase in blood LH levels for those LH pulses that did occur during stimulation. These results indicate that peri-SCN-induced suppression of pulsatile LH release is not mediated by known DA and/or 5-HT neuronal inputs to this region, and suggest that the peri-SCN area may be one brain region that influences, in a suppressive manner, only the periodicity of the pulsatile LH release process in the ra