The development of hypertension was studied in rats after neonatal sympathectomy by s.c. injection of 6-hydroxydopamine HCl. Three different types of hypertension were investigated: renal hypertension in the two-kidney Goldblatt model, steroid hypertension produced by deoxycorticosterone (DOCA) implantation and saline as drinking fluid, and genetic hypertension in the spontaneous hypertensive rat (SHR). Blood pressure was measured directly in conscious animals via the iliac artery. Mean blood pressure of conscious sympathectomized (SX) normotensive rats was not significantly different from that of normotensive controls. Renal hypertension reached the same level in controls and SX rats four weeks after application of a renal artery clip. DOCA-salt hypertension developed faster and to a higher level in SX rats than in control rats. The hypertension in SX DOCA-salt hypertensive rats was accompanied by a marked tachycardia. In contrast hypertension did not develop in SX SHR. Up to 12 months of age mean blood pressure was markedly lower than that of control SHR, but slightly higher than that of SX normotensive Wistar Kyoto rats. These differential effects of neonatal sympathectomy on the development of hypertension in the rat may point to differences in the pathophysiological mechanisms. It is concluded that an intact sympathetic nervous system is essential for the development of hypertension in SHR. In DOCA-salt hypertension the intact sympathetic nervous system appears to protect against a rapid rise in blood pressure.