Comparisons were made regarding the effects of 50 Hz bilateral electrical stimulation at either 100 or 200 μA applied to the preoptic-suprachiasmatic (POA-SQ) or the arcuate-ventromedial n. (ARC-VMH) regions on prolactin (Prl) and LH plasma levels in male rats under pentobarbital anesthesia. No significant differences were observed in Prl levels between samples obtained either by decapitation, by heart puncture 1 h after pentobarbital administration or by cannulation and sequential sampling with blood cells replacement under pentobarbital anesthesia (35 mg/kg, i.p.)·similar results were obtained with LH. Hormonal values in animals subjected to stereotaxic procedures and electrode penetration (sham controls) had slight but significantly higher Prl levels than the rest of the control groups. LH levels tended to be higher at the end of the experimental period. Stimulation of structures within the POA-SQ region with 100 μA elicited a significant increase by 30 min, with values decaying towards control levels by the end of the experimental period. A similar stimulus applied to the ARC-VMH region failed to promote significant changes in Prl levels. LH plasma values were increased by stimulation applied to either region. With 200 μA applied to the POA-SQ, Prl levels rose higher than with 100 μA and dropped rapidly thereafter. LH values were also significantly increased but levels remained high until the end of the experimental period. Stimulation of the ARC-VMH induced a significant Prl release with high levels persisting during the rest of the experiment, thus giving a response curve different to that one obtained after stimulation of the POA-SQ. LH levels rose to values comparable to those obtained after 100 μA stimulation. It is concluded that the medial preoptic area (MPO), periventricular n., suprachiasmatic n., ventral portions of the anterior hypothalamic area (AHA) contain elements of a neural circuit involved in the control of Prl release in the male rat; that elements within this circuit (as related to Prl control) are apparently different from those in the ARC-VMH (mainly ventral portion of the ventromedial n. and arcuate n.) and that acute conditions do not seem to be as restrictive as in the female rat to obtain Prl release after POA-SQ stimulation. Regarding LH control mechanisms, results are suggestive that neural elements stimulated within the POA-SQ and ARC-VMH regions eliciting LH release belong to the sam