Prolactin (PRL) secretion patterns were determined in freely moving male and female rats chronically fitted with electrodes in the sexually dimorphic component of the medial preoptic area (MPOA) and the median eminence (ME), together with a chronic atrial blood sampling catheter. Electrodes made of stainless steel or platinum-iridium were implanted bilaterally. Female rats with 4 electrodes in the ME exhibited an attenuated surge of PRL secretion on proestrus (injured group), rats with 2 electrodes in the ME had a normal proestrous surge (intact group). Basal PRL levels were not different between the two groups. Stimulation (100-Hz voltage pulses, 0.2 ms width, 10 s on/5 s off, total duration 25 min) did not produce lesions. Evidence is presented that experimentation was performed without stress. Electrical stimulation applied to the MPOA increased PRL secretion in males, decreased PRL secretion in proestrous females and was without effect in conscious or anesthetized diestrous females. Electrical stimulation applied to different locations of the ME was without effect in male and diestrous female rats; it reduced PRL secretion in proestrous females. These data demonstrate that the MPOA has a sexually differentiated function in the regulation of PRL secretion. The data do not provide evidence for a sexual dimorphism in prolactin-inhibiting factor and prolactin-releasing factor activity in the ME.