In 34 hypothalamic slices of rats, spontaneous discharging of 63 paraventricular neurons was extracellulariy recorded by glass microelectrode. When the slices were perfused with artificial cerebrospinal fluid (ACSF) containing noradrenaline (NA) (10-6 mol/L), firing rates of 20 units significantly increased, those of 8 units decreased or even ceased, and those of 35 units, not affected. When low Ca2+ high Mg2+-ACSF was applied to block synaptic transmission, of 20 units which were excited by NA, 14 units still had excitatory responses and 6 units had no significant response to NA. When synaptic transmission was blocked, of 8 units which were inhibited by NA, 7 units still had inhibitory responses and only one unit had no significant response to NA. The results of this experiment strongly suggest the existence of direct effects of NA on paraventricular neurons.