1. The pharmacokinetics of cyclosporin were studied in six renal transplant patients, following intravenous administration of 5 mg kg-1 infused over 3, 6 and 24 h. 2. Plasma, separated at 37 degrees C, was analysed for cyclosporin by h.p.l.c. 3. The data were described by a biexponential disposition model. 4. None of the disposition parameters (clearance, initial volume of distribution, half-lives) changed with the duration of infusion.