Recordings were made of single unit activity through chronically implanted electrodes within the hypothalamic paraventricular nucleus of freely moving male rats also bearing chronically implanted stimulating electrodes in the preoptic area (POA) and indwelling jugular cannulae. High frequency electrical stimulation of the POA evoked increases in the rate of electrical discharge in 58% of the units tested. Increases were recorded during each period of stimulation and were subsequently followed by a significant decrease in the rate of firing. In a substantial proportion of these neurones, bursting patterns of electrical discharge were observed before, during, and following stimulation of POA. When simultaneous removal of blood samples from these animals was performed, a significant increase in plasma corticosterone concentration, immediately following POA stimulation, was observed. The data obtained have provided a possible neurophysiological correlate of hypothalamic electrical activity that may be directly related to the neural control of adrenocortical secretion.