Renal elimination of the immunosuppressant ciclosporin is virtually unknown. Therefore, in 17 renal allograft recipients under steady-state conditions we studied the urinary excretion of ciclosporin and 17 of its metabolites in blood and 24-hour urine. Patients with liver dysfunction or treated with drugs potentially influencing the metabolism and elimination of ciclosporin were excluded from the study. Ciclosporin and its metabolites were measured by HPLC. Metabolite but not ciclosporin excretion was strongly correlated with creatinine clearance. Metabolites 18 and 26 (β, ε-cyclic metabolite) were rarely found in blood but were excreted in considerable amounts in urine. Approximately 3% of the administered dose of ciclosporin per day undergoes renal elimination in unchanged form or as metabolites investigated. The data suggest glomerular filtration of ciclosporin metabolites, a difference in the rate of elimination between ciclosporin and the metabolites and some kind of metabolism or active transport mechanism for metabolites in the kidney.