Pharmacokinetic interaction of theophylline with pefloxacin following i.v. administrations was investigated in rabbits. Pefloxacin was coadministrated at doses of 10 and 20 mg/kg or previously administered for 6 days 10 and 20 mg/kg. Plasma concentration and AUC of theophylline were increased significantly (p<0.05) and the renal clearance $(CL_r)$, total body clearance $(CL_r)$ and the volume of distribution $(Vd_{ss})$ were decreased significantly (p<0.005) by the pretreatment. It demonstrates that adjustment of dosage regimen of theophylline should be considered when concomitant administration of pefloxacin is prescribed.