The substance and tablets of pefloxacin mesilate manufactured by the Urals Polytechnical University and the National Research Centre of Antibiotics were studied on rats and dogs with the drug oral administration in single and multiple doses equivalent to the human ones. The bioavailability of the drug was good and its blood levels were efficient for a prolonged period (at least 24 hours) after a single oral administration. No cumulation of the antibiotic after its repeated use was observed. The pefloxacin mesilate tablets proved to be fully equivalent to the drug manufactured by Rhone-Poulenc-Rorer (France).