Plasma concentrations of angiotensin II (A II), aldosterone (Aldo) and arginine vasopressin (AVP), and serum osmolality (Sosm) were determined before and after gradually increasing exercise loads on a bicycle ergometer in 10 hypertensive (group I) and 10 normotensive renal transplant recipients (group II), and in 15 healthy control subjects (group III). Working capacity was reduced in groups I and II. The A II, Aldo, AVP, Sosm increased in all groups after exercise. The A II was higher in group I than II and the percentage changes were significantly lower in groups I and II than in group III. There were no significant differences in Aldo between the groups either before or after exercise. The AVP was the same in groups I and II, and AVP in these groups was higher than in group III. The Sosm and AVP were significantly correlated in all groups. Neither A II, Aldo nor AVP were significantly correlated to systolic blood pressure (BP). Alterations in AVP, but not in A II or Aldo, were correlated to the degree of exercise load. It can be concluded that the renin-angiotensin-aldosterone system and the osmoregulatory system are stimulated during exercise in renal transplant recipients. The A II is elevated in post-renal transplant hypertension, but the responsiveness is reduced in both hypertensive and normotensive recipients. The alterations in AVP are probably secondary to changes in Sosm, and the higher AVP levels in recipients could be due to a decreased responsiveness of the renal tubules to AVP. Our findings are in good agreement with the hypothesis that hypertension after renal transplantation is angiotensin II-dependent.