The effects of early sympathectomy on the development of salt hypertension were studied in prepubertal and adult rats with hereditary diabetes insipidus (DI). Early guanethidine administration caused a pronounced and long-term destruction of sympathetic nervous system (SNS) in Brattleboro rats in which blood pressure (BP) was significantly decreased until the age of 22 weeks. This SNS impairment did not abolish the age-dependent BP response of salt-loaded rats that was still greater in young than in adult sympathectomized DI rats. BP of young uninephrectomized DI rats was higher in the late than in the early phase of salt hypertension development. The early sympathectomy lowered BP and increased mortality in all groups of saline drinking DI rats except young uninephrectomized animals in which hypertensive response was attenuated but not prevented. It could be suggested that 1) increased BP response of young rats to high salt intake occurs even in animals with attenuated principal pressor systems, 2) the effects of early sympathectomy on the development of salt hypertension depend on the actual hemodynamic pattern, and 3) moderate BP increase might be a part of homeostatic mechanisms defending the organism threatened by chronic salt overload.