The effects of norfloxacin (NOR), at steady-state plasma concentrations of 0-32 mg.l-1, on the plasma clearance of a 6 mg.kg-1 i.v. bolus dose of theophylline (THEO) in the male Sprague-Dawley rat have been studied. The effects were characterised by a Ki value (Ki = 12 microM), which was comparable with Ki values obtained previously under identical conditions for ciprofloxacin, but higher than that obtained for enoxacin. The distributional characteristics, volume of distribution and liver to plasma concentration ratio, were very similar for the three compounds. The only marked pharmacokinetic differences were in hepatic clearance, where there was a rank order NOR > ciprofloxacin > enoxacin, a reverse of the order in the reduction of THEO clearance seen in clinical studies. The advantages of using the steady-state experimental design described here are that equivalent concentrations are utilised to compare related drugs and differences in pharmacokinetics are accounted for, to allow a direct comparison of potency. This information, together with additional pharmacokinetic considerations, suggests that the different effects on THEO clearance seen in the clinic for NOR, ciprofloxacin and enoxacin are not solely due to differences in inhibitory potency, but also involve differences in hepatic clearance and hence systemic availability of the fluoroquinolones.