Eleven patients, who had undergone renal transplantation and who had hypertension, aged 19-56 years, were treated with cyclosporine and prednisolone. We measured plasma renin activity, aldosterone and vasopressin (RIAs) at the first, second and third week and again 9 to 12 months after transplantation. Plasma renin activity was in the low-normal range throughout (0.31 +/- 0.05, 0.30 +/- 0.03, 0.32 +/- 0.05 ng/ml/h on short- vs. 0.32 +/- 0.04 ng/ml/h on long-term), aldosterone showed a tendency to decrease (114 +/- 27, 72 +/- 18, 71 +/- 11 pg/ml on short- vs. 54 +/- 23 pg/ml on long-term), whereas vasopressin remained moderately increased during the observation period (10.5 +/- 0.8, 10.4 +/- 1.6, 8.9 +/- 0.6 pg/ml on short- vs. 9.6 +/- 1.0 pg/ml on long-term). We then investigated the reactivity of the renin-system in 5 of the patients by stimulating renin release by captopril. Increases in plasma renin activity were only moderate (0.35 +/- 0.03 vs. 0.66 +/- 0.21 ng/ml/h) and blood pressure dropped only slightly (148 +/- 2.0/98 +/- 1.2 vs. 141 +/- 4.6/95 +/- 4.2 mmHg). Levels of plasma aldosterone were significantly suppressed from a low baseline (46.4 +/- 13.5 vs. 25.3 +/- 6.1 pg/ml, p less than 0.05). The increase in vasopressin was unaffected by captopril (9.6 +/- 1.0 vs. 8.8 +/- 0.4 pg/ml). Our results suggest that in renal transplantation patients with good graft function, the activity of the renin system is unaffected by cyclosporine treatment on short- and on long-term. Vasopressin stimulation does not seem to depend on the renin system and might play a role as a vasoconstrictor in the face of a denervated kidney.