Variations of serum prolactin (PRL) levels and activities of tuberoinfundibular dopaminergic (TIDA) neurons during the afternoon of ovariectomized (OVX) rats treated with or without estrogen were determined in this study. Long-term OVX rats treated with or without polyestradiol phosphate (0.1 mg/rat, s.c.) were decapitated every hour from 10.00 to 19.00h(except 11.00 and 13.00 h). Serum PRL and median eminence (ME) dihydroxyphenylacetic acid (DOPAC) or dihydroxyphenylalanine (DOPA) levels were determined by radioimmunoassay and high performance liquid chromatography plus electrochemical detection, respectively. A prominent PRL surge started and peaked around 14.00–15.00h, and remained significantly higher than levels of 10.00 and 12.00 h throughout the afternoon. Significant decreases of ME DOPAC and DOPA concentrations were also observed between 14.00 and 19.00 h. In OVX rats with no estrogen replacement, no PRL surge was observed and the changes of ME DOPAC concentrations during the afternoon were not significant except for that at 17.00 h. The ME DOPA accumulation, however, exhibited significantly lower levels from 14.00 to 19.00 h than that at 12.00 h, indicating that an endogenous rhythm for DA synthesis existed in OVX rats. In estrogen-treated OVX rats bearing bilateral lesions of the suprachiasmatic nuclei, both changes in serum PRL level and TIDA neuron activity were abolished. We conclude that an endogenous rhythm of the activities of TIDA neurons may exist in both OVX and OVX plus estrogen-treated rats. The rhythm is regulated by the suprachiasmatic nuclei and may be amplified by estrogen for the induction of PRL surg