To determine a change in theophylline pharmacokinetics during concomitant thiabendazole or mebendazole therapy, we studied six normal, healthy male volunteers. Aminophylline was administered intravenously, followed by a 30-h blood sampling period. Subjects were randomized to receive thiabendazole or mebendazole, then crossed over to receive the other therapy. Theophylline concentrations were measured utilizing an HPLC technique and a one-compartment model was fit to the data. Theophylline pharmacokinetic parameters were significantly different during thiabendazole therapy. Mean theophylline half-life increased, clearance decreased and elimination rate constant decreased. Two subjects experienced severe nausea and vomiting during thiabendazole therapy. There were no significant differences in theophylline pharmacokinetic parameters during mebendazole therapy. Thiabendazole administration results in a significant decrease in theophylline clearance and beta elimination rate constant. The theophylline half-life increased significantly. Concomitant administration of theophylline and thiabendazole resulted in severe nausea and vomiting. Mebendazole administration did not seem to alter theophylline pharmacokinetics.