The expression of Fos-related antigens (FRAs) in the A<sub>[2</sub> and the A<sub>14</sub> hypothalamic dopaminergic neurons was compared throughout the day in proestrous (PRO) and ovariectomized (OVX) rats to establish the relationship between secretion of prolactin (PRL) and dopaminergic neuronal activation. Animals with intact ovaries were sacrificed at 11:00, 13:00, 15:00, 17:00 and 21:00 h on the day of proestrus and 06:00 and 09:00 h in the morning of estrus. OVX animals were sacrificed at the same time points on the 12th day after surgery. Double-label immunocytochemistry was performed by using antibodies against FRAs as markers of tonic neuronal activity and tyrosine hydroxylase to identify dopaminergic neurons. Serum PRL levels were determined by radioimmunoassay. A pattern of FRAs expression was present in the A<sub>14</sub> and A<sub>12</sub> neurons of PRO rats. The incidence of FRAs expressing neurons was the highest in the first half of the day when the PRL levels were low, and decreased prior to the surge of serum PRL during the afternoon. This pattern was present in the dopaminergic neurons of the periventricular nucleus (A<sub>12</sub>), and in all portions of the arcuate nucleus (A<sub>12</sub>) except the ventrolateral portion of the middle arcuate nucleus. A similar pattern of FRAs expression existed in the A<sub>14</sub> and A<sub>12</sub>neurons of OVX rats in spite of that there were no detectable changes in serum PRL levels. However, the amplitude of decrease in the incidence of FRAs-labeled neurons was lower in OVX than in PRO rats. These data suggested that similar to the control of hypothalamic PRL releasing factors, the activation/deactivation pattern of the A<sub>12</sub> and the A<sub>14</sub> neurons is governed by a seemingly endogenous rhythmic input. The incidence of this rhythm is independent of the reproductive state, but its amplitude is enhanced by ovarian steroids. The disparity between the FRAs expression and serum PRL levels in OVX rats indicated that the effect of this endogenous dopaminergic rhythm upon PRL secretion is dependent on the ovarian steroid background.