Kinetics of the novel immunosuppressive cyclosporin were determined in 4 patients with terminal renal failure. After a short i.v. infusion (2.05-3.5 mg/kg in 4 h), blood and plasma concentrations were measured (HPLC [high performance liquid chromatography] and radioimmunoassay [RIA]) up to 36 h. After infusion, concentration curves of the drug were characterized by a rapid initial fall (t1/2.alpha. [.alpha.-phase half-life] 0.10 .+-. 0.03 h), followed by a biphasic elimination phase with corresponding t1/2s of 1.08 .+-. 0.25 h (t1/2.beta. and 15.8 .+-. 8.4 h (t1/2.gamma.). The volumes of distribution, calculated from whole blood concentrations (HPLC), were 0.140 .+-. 0.48 l/kg (volume of the central compartment) and 3.49 .+-. 2.65 l/kg (volume of distibution at steady state), whereas systemic clearances were 0.369 .+-. 0.08 l/h per kg. Blood levels measured by RIA exceeded the HPLC values after the 4th h by up to 100%, indicating the production of cross-reactig cyclosporine metabolites. Plasma concentrations were considerably lower than in whole blood. Elimination of unchanged cyclosporine in patients with renal failure appears to be of the same order as in those with normal kidney function. Modification of the initial dosage regimes is probably not required.