Due to the retarded elimination of azathioprine and its metabolites immunosuppressive therapy creates certain problems during acute or chronic renal failure. For this reason the pharmacokinetics of azathioprine and its metabolites were studied in patients with chronic renal failure. Elimination during eight hours of haemodialysis amounted to 45%, which is only slightly less than the elimination in the urine of normal persons over a period of 24 hours. The dialysance of azathioprine and its metabolites was similar to that for uric acid. Effective elimination through haemodialysis is therefore possible. We are of the opinion that the dose of azathioprine for effective immunosuppressive therapy can be maintained under simultaneous haemodialysis.