When GABA (4-amino-n-butyric acid, 50-200 micrograms) was injected into the lateral ventricle of urethane-anaesthetized Wistar rats, sympathetic nerve activity, arterial pressure and heart rate were decreased dose-dependently. Graded electrical stimulation of the ventromedial hypothalamus (50, 100 and 150 microA) increased not only mean blood pressure but also the rate of sympathetic nerve firing, and both responses were attenuated by GABA pretreatment (100 and 200 micrograms, i.c.v.). In spontaneously hypertensive rats (SHR), i.c.v.-injected GABA also reduced sympatho-cardiovascular activity, but the magnitude of the depressor responses was significantly larger in SHR than in normotensive Wistar-Kyoto (WKY) control rats. Pressor and sympathetic nerve responses elicited by hypothalamic stimulation were initially larger in SHR than in WKY rats. However, upon subsequent i.c.v. injection of GABA, hypothalamic responsiveness in SHR was inhibited more prominently and became almost the same as that in WKY rats. These results suggest that, by depressing hypothalamic function, central GABA-ergic stimulation decreases sympathetic nerve activity thereby lowering blood pressure and heart rate. Because of the increased central sensitivity in SHR, GABA-ergic stimulation reversed hypothalamo-sympathetic hyperactivity and attenuated hypertension.