To examine tbe role of rasopressin (VP) in DOCAhyphensalt hypertension, arterial pressure and renal vascular reactirity were studied in control Longhyphenerans (LE) rats and in Brattleboro rats homozygous for diabetes insipidus (DI rats). Vascular reactirity to norepinephrine, VP and angiotensin II was assessed in isolated kidneys perfused at constant flow. LE rats snowed an increase in arterial pressure (AP) which was significant at 2 weeks post DOCA and averaged 180 mm Hg at 4 weeks. DI rats lacking VP showed no rise in AP after DOCA; however, DI rats given VP and DOCA developed hypertension with a course and magnitude similar to that observed in LE rats. At 6 to 10 weeks post DOCA, renal vascular reactivity to all agents was increased in LE rats and DI rats replaced with VP. Nonnotensive DI rats lacking VP snowed depressed reactivity. Assessment of changes in reactirity at 3 days post DOCA showed that changes preceded tbe rise in AP. These data suggest that VP may play a primary role in the patbogenesis of DOCA hypertension and that its mechanism may involve an induction of increased vascular reactirity. (Hypertension 4: 3–12, 1982)