We investigated the role of vasopressin, angiotensin II, and catecholamines in the onset of acute (45-minute) aortic coarctation hypertension in conscious rats. Partial aortic constriction was performed by means of a pneumatic cuff placed around the abdominal aorta above the renal arteries for 15 or 45 minutes. A sham-operated group was used as control. Mean carotid pressure before aortic constriction did not differ between rat groups. Aortic constriction produced a similar increase of mean carotid pressure during 15 minutes (36±3 to 37±3 mm Hg above basal levels) and 45 minutes (37±2 to 39±3 mm Hg). Plasma vasopressin concentration after 15 minutes of coarctation (4.4 ±0.5 pg/mL) did not differ from that observed in control rats (3.0±0.8 pg/mL), whereas after 45 minutes, it was significantly higher (14.3±3.3 pg/mL). Plasma renin activity increased significantly after coarctation (21.7±4.1 and 29.9±2.9 ng angiotensin I/mL per hour, at 15 and 45 minutes, respectively) when compared with control rats (3.9±0.5 ng angiotensin I/mL per hour). After coarctation, plasma norepinephrine concentration was consistently reduced, whereas plasma epinephrine concentration did not differ from control rats. In conclusion, these data provide evidence for an effective vasopressor role for vasopressin in the genesis of acute (45-minute) aortic coarctation hypertension in conscious rats. In addition, although the results confirm that the renin-angiotensin system participates earlier in the onset of coarctation hypertension, they rule out a significant vasopressor role for catecholamines in the early development of hypertension.