Hypertension is common after orthotopic liver transplantation and may be due, in part, to cyclosporin A-induced renal dysfunction and/or enhanced proximal tubular sodium reabsorption. To determine whether enhanced proximal tubular sodium reabsorption is central to the development of posttransplant hypertension, measurements of renal hemodynamics and fractional clearances of lithium and sodium were compared 1 month after orthotopic liver transplantation in previously normotensive patients receiving either cyclosporin A (N = 24) or FK506 (N = 18), an immunosuppressive agent that is structurally unlike cyclosporin A and that has a lower reported incidence of hypertension. Median prednisone doses were 20 and 13 mg/day in the cyclosporin A and FK506 groups, respectively (P < 0.05). At 1 month, mean arterial blood pressure was higher in the cyclosporin A versus the FK506 group: 108 +/- 2 versus 95 +/- 3 mm Hg (P < 0.05). GFR, RBF, and renal vascular resistance were not different between the two groups: 59 +/- 4 and 53 +/- 5 mL/min per 1.73 m2, 439 +/- 28 and 440 +/- 41 mL/min per 1.73 m2, and 22,429 +/- 1,822 and 22,977 +/- 3,506 dyne s/cm5 per 1.73 m2, respectively. Fractional lithium excretion was similar in the cyclosporin A and FK506 groups: 19.9 +/- 2.2 and 19.4 +/- 2.0% (P = not significant) although both values were lower than those of normal controls (25.5 +/- 1.1%) (P < 0.05). Fractional sodium excretion was 2.7 +/- 0.3 and 2.3 +/- 0.4% in the cyclosporin A and FK506 groups, respectively (P = not significant).(ABSTRACT TRUNCATED AT 250 WORDS)