The renal dysfunction induced by cyclosporine (CsA) has been demonstrated to be at least partially reversible after cessation of CsA therapy. The time course and magnitude of changes in various parameters of renal function after CsA withdrawal have not been studied in detail. We examined 12 renal transplant patients immediately before and 1, 2, and 4 weeks after replacement of CsA by azathioprine at 3 months after transplantation. Nine patients in whom CsA was continued during this period served as a control group. A significant increase in glomerular filtration rate ([GFR] 15 + or - 17%, p <0.01) occurred already in the first week after discontinuation of CsA. From 1 to 4 weeks after conversion, GFR did not significantly increase any further. A fall in serum creatinine (-7 + or - 9%, Ns) paralleled the rise in GFR (r = -0.76, p <0.01), but there was a further decrease of creatinine in the second to fourth week after conversion. Withdrawal of CsA induced a rise in serum magnesium in all patients (0.73 + or - 0.13 vs 0.86 + or - 0.12 mmol/l, p <0.001) as well as a marked decrease in the serum level of urate (0.39 + or - 0.09 vs 0.33 + or - 0.07 mmol/l, p <0.01) within one week. None of the observed changes took place in the control group. In conclusion, a major improvement of GFR occurs within one week after cessation of CsA therapy. Changes in the serum levels of magnesium and urate appear to be the most responsive markers of the renal effects of CsA.