Extracellular electrical activities of paraventricular nucleus (PVN) cells were recorded in anaesthetized adult rats by using glass microelectrodes. After antidromical identification of the magnocellular neurosecretory cells, the effects of intracerebroventricular (i.c.v.) injection of norepinephrine (NE) on such cells were observed. Of 49 neurons of antidromic stimulation, phasic, fast continuous and slowly irregularly. firing units constituted 42.9%, 36.7%, and 20.4% of the total population respectively. With i.c.v. injection, NE inhibited the activity of most phasically firing units, but enhanced that of most fast continuous and slowly irregular firing units. The inhibition effect of NE could be partially antagonized by i.c.v. injection of phentolamine (2 micrograms/microliters), showing that the effect is possibly mediated by alpha-receptor. The above findings indicate that NE plays different roles in different PVN magnocellular neurons having different patterns of firing activity.