The pharmacokinetics of vidarabine were studied in 8 patients with polyarteritis nodosa related to hepatitis B virus infection. The drug was administered by continuous infusion for three weeks at doses of 15 (1 week) and 7.5 (2 weeks) mg/kg per day, during which time 15 plasma exchanges were performed. Plasma was assayed for vidarabine and its principal metabolite, hypoxanthine arabinoside by high pressure liquid chromatography. Vidarabine was not detected in the plasma of any patients. Hypoxanthine arabinoside levels were used to evaluate vidarabine kinetics. The serum levels of hypoxanthine arabinoside ranged from 3.6 to 21.5 mg/l. The mean elimination half-life (+/- SD) was 3.0 +/- 1.7 h. The plasma clearance (mean +/- SD) was 195 +/- 270 ml/min when the dose was 7.5 mg/kg per day and 66.3 +/- 47 ml/min for a 15 mg/kg per day/dose (NS). Except for the elimination half-life, these results were not fully consistent with those observed in other studies. The influence of multiple plasma exchanges on vidarabine kinetics is limited and dosage adjustment is not required based on the continuous infusion of vidarabine.