A diurnal change of the tuberoinfundibular dopaminergic (TIDA) neuron activity in ovariectomized rats treated with or without estrogen was recently reported, and the endogenous cholinergic system may be responsible for its induction. Whether a similar rhythm exists in intact female or male rats was the focus of this study. TIDA neuron activity was assessed by measurement of the precursor or metabolite of dopamine in the median eminence by HPLC with electrochemical detection. In intact female Sprague-Dawley rats, diurnal changes in TIDA neuron activity were observed during all stages of the estrous cycle, i.e., proestrus, estrus, and diestrus 1, and they were nearly identical. No such rhythm, however, was observed in intact male rats. Castration alone decreased and increased basal TIDA neuron activity in female and male rats, respectively, and estrogen treatment increased the activity in both sexes. The diurnal changes in TIDA neuron activity, however, were observed only in the female rats, not the male rats, regardless of castration or estrogen treatment. Serum prolactin levels in the male rats exhibited no diurnal changes either, irrespective of whether the animals had been castrated or treated with estrogen. Central administration of mecamylamine (1 micrograms/3 microliters per rat, i.c.v.), a nicotinic receptor antagonist, in the morning (at 1000 h) or afternoon (at 1500 and 1700 h) induced a differential effect on the TIDA neuron activity of intact female rats, i.e., no effect in the morning and stimulation in the afternoon; it had no effect on the TIDA neuron activity in the male rats regardless of the injection time. In castrated male rats treated with estrogen, however, mecamylamine treatment further increased TIDA neuron activity, but still with no diurnal difference. In summary, a sexual difference in the diurnal changes of TIDA neuron activity was observed, and these changes may be under differential control by the cholinergic system.