The simultaneous saliva and plasma ofloxacin (OFLX) concentrations in patients with respiratory tract infection were measured by a microbiological agar-well diffusion assay following a single oral administration of an OFLX preparation. The maximum concentration (Cmax), the time to reach that peak (tmax) and the elimination half-life of OFLX derived from salivary concentration data were 5.2 +/- 0.4 mg/l, 1.9 +/- 0.4 h and 3.6 +/- 0.5 h, respectively, which were in agreement with those derived from plasma concentration data. The mean saliva-to-plasma concentration ratio (Cs/Cp) obtained after the administration of a single oral dose was 1.08 +/- 0.05. This was both time-independent and concentration-independent, and it showed a certain relationship with age. A significant correlation was obtained between plasma and saliva concentrations (r = 0.768, p less than 0.01). The coefficient of variation for the overall variability was approximately 30.0%. The present results indicate that the secretion of OFLX into saliva may be, at least, due to a passive diffusion mechanism, but a measurement of OFLX concentration in saliva as a guide to dose adjustment would be too wide at odds with plasma concentration to form the basis for a clinical decision.