The role of the paraventricular nucleus of the hypothalamus (PVH) in the development of hypertension was determined after bilateral electrolytic or sham lesions of this structure in 4-5-week-old male spontaneously hypertensive rats (SHR). The average arterial pressure in the PVH-lesioned group was significantly lower compared to sham-lesioned animals during the first 3 weeks after the PVH lesions. At 9 weeks of age the arterial pressures of the PVH-lesioned animals increased, but remained significantly lower than those of the sham-operated animals of the same age. This difference in arterial pressures was observed to 16 weeks of age. Heart rate was significantly reduced by PVH lesions up to 5 weeks after the lesions, at which point the heart rate tended towards the control values of the sham-lesioned animals. These data have demonstrated that the region of the PVH is important in the initial phase of the development of hypertension and in the full expression of the hypertension in the SHR, and provide evidence of a central mechanism in the hypertensive process in the SHR.