We have established a totally-immersed, perfused slice preparation of the hypothalamus which is amenable for electrophysiological and pharmacological studies. The amount and pattern of spontaneous activity in the paraventricular nucleus (PVN) is markedly influenced by varying the amount of Ca++ in the oxygenated physiological medium which continuously perfuses the slice over both upper and lower surfaces. Ca++ concentrations greater than 1 mM virtually abolish spontaneous activity, although the neurons discharge in response to advance of the electrode and are activated by addition of glutamate to the perfusate. However, in a perfusing medium containing 0.75 mM Ca++, most cells display 1-7 Hz spontaneous activity for up to 10 h; some cells display phasic activity similar to that attributed to vasopressin neurons in vivo. Electrical stimulation peripheral to the PVN elicits antidromic potentials in some PVN neurons, sometimes followed by a post-activation depression of activity typical of recurrent inhibition. Under appropriate perfusion conditions, therefore, the hypothalamic slice preparation displays characteristics of the in vivo hypothalamus.