Based on the results of a previous study which demonstrated a 50% reduction in theophylline clearance during coadministration of 400 mg of enoxacin twice a day (b.i.d.), a sequential-design study was completed with seven nonsmoking, healthy adult female human volunteers. The subjects were given 200 mg of theophylline (Theo-Dur) orally every 12 h for 4 days. On day 5, the subjects began receiving 400 mg of enoxacin with each theophylline dose, and the dosage of theophylline was reduced to 100 mg b.i.d. This regimen was continued through day 8, after which enoxacin was discontinued. The theophylline dosage was increased to 200 mg b.i.d. on day 9, and theophylline monotherapy continued through day 12. The mean apparent theophylline clearance decreased by approximately 50% during enoxacin coadministration. No significant differences in mean theophylline maximum concentration in serum, time to maximum concentration in serum, lowest concentration observed, or area under the concentration-time curve during the steady-state dosing were observed before, during, or after enoxacin coadministration when the theophylline dosage was reduced to 100 mg b.i.d. Reduction of the theophylline dose by 50% at the onset of enoxacin dosing maintained constant theophylline concentrations in plasma. A return to the original theophylline dose immediately upon cessation of enoxacin therapy resulted in a transient 35% increase in theophylline concentrations in plasma which lasted 24 to 48 h before returning to preenoxacin values. Although a 50% reduction in the theophylline dose maintained constant mean theophylline concentrations when enoxacin was administered concomitantly, it appears that larger dose reductions (up to 75%) could be required in patients with high theophylline clearances. In addition, larger transient increases in the theophylline concentration in plasma may be observed in these patients upon cessation of enoxacin therapy if the theophylline dose is immediately returned to normal. Thus, it is recommended that theophylline concentrations in plasma be monitored when concurrent enoxacin therapy is required.