The effect of electrical stimulation in the subfornical organ (SFO) on plasma luteinizing hormone (LH) concentrations has been examined in the conscious freely moving male Sprague-Dawley rat. Plasma LH levels were significantly elevated above basal levels 15 min following stimulation in the SFO. In contrast, stimulation in the adjacent hippocampal commissure caused no significant change in plasma concentrations of LH. These results correlate well with previous electrophysiological studies demonstrating that SFO stimulation activates putative LH-releasing hormone (LH-RH) containing neurons in the septum which project to the median eminence and add further support to the hypothesis that the SFO plays a significant role in the control of reproductive function.