To account for the individual variability in cyclosporine pharmacokinetics and the non-existence of dosing recommendations in young children, we studied the pharmacokinetics of cyclosporine before renal transplantation in ten children aged 1.1-2.5 years, to determine the appropriate individual dose. Our aim was to reach a steady-state cyclosporine blood level of 200-300 micrograms/l, 8 h after a dose in the first days after renal transplantation. Cyclosporine was given as a single oral dose (10 mg/kg) or as a 4-h i.v. infusion (3 mg/kg), and the blood concentration was determined for 24 h by a specific monoclonal radioimmunoassay. The mean terminal cyclosporine half-life (t1/2) was 9.3 h (range 2.8-20.4), blood clearance 10.8 ml/min per kilogram (range 6.8-22.7) and volume of distribution 2.8 l/kg (range 1.4-4.7). The bioavailability of oral cyclosporine was low; the mean amount absorbed was 21.8% of the administered dose (range 11-35). The mean calculated dose needed to attain the intended predose blood cyclosporine level of 200-300 micrograms/l at steady-state was 5 mg/kg per day for i.v. and 21 mg/kg per day for oral administration. In view of the short t1/2, we used three doses/day. The validity of the predicted doses is shown by the mean cyclosporine doses used during the first 10 days after transplantation, which were 93.5% of the calculated oral and 96.6% of the calculated i.v. doses. The observed mean cyclosporine concentration during the same period was 196 micrograms/l.