In fourteen renal transplant patients converted from cyclosporin to azathioprine for financial reasons renal blood flow was greater and renal vascular resistance lower after conversion. These changes occurred without any change in serum creatinine. This finding suggests that the renal allograft vasoconstriction induced by cyclosporin occurs before evidence of nephrotoxicity and that the vasoconstriction is reversible. Renal allograft recipients had cyclosporin-induced haemodynamic changes even early (2 weeks) after transplantation, when the allograft is still denervated. These findings may explain the increased sensitivity to other nephrotoxic agents and hypertension observed in cyclosporin-treated patients.