The pharmacokinetics of the quinolone pefloxacin were determined following a 400 mg oral dose given to each of six male volunteers. Concentrations were determined in serum and urine by high-performance liquid chromatography, and in cantharidin-induced inflammatory fluid by a microbiological assay. The mean peak serum level of 6.6 micrograms/ml was attained rapidly 0.8 h after administration. The mean serum elimination half-life was 11.6 h. Inflammatory fluid was penetrated quickly with a mean peak level of 3.9 micrograms/ml occurring at 2.4 h. Pefloxacin was excreted in the urine as the parent compound and its two metabolites, norfloxacin and pefloxacin N-oxide (24 h urinary recovery being 8.0%, 12.0% and 13.1% respectively of the dose). This study suggests that a twice or possibly once daily dosage may be sufficient to treat systemic infections caused by susceptible pathogens. Once daily dosing should be sufficient for urinary tract infections.