Анотация: Enrofloxacin, a fluoroquinolone drug, is often used in treatment of bacterial infections in dogs because of its potent bactericidal activity and wide distribution in tissues. The aim of the present study was to determine the penetration of enrofloxacin and its main active metabolite ciprofloxacin in aqueous humour (AH) by means of population pharmacokinetics. A single dose of enrofloxacin 5 percentage sterile solution for injections (Baytrilо, Bayer HealthCare) was administered subcutaneously to 24 dogs at a dose rate of 7.5 mg/kg. Concentrations of enrofloxacin and its main metabolite ciprofloxacin in plasma and AH were analyzed by high performance liquid chromatography. Enrofloxacin and ciprofloxacin were found in the studied fluids 24 h after parent drug administration. Maximum concentrations were lower in AH compared to plasma. The absorption rate constant of enrofloxacin was lower for AH (0.12 h-1) than for plasma (0.62 h-1). A lower value for this constant was obtained for the main metabolite ciprofloxacin: 0.07 h-1 for AH. The data from the current study with healthy animals showed that enrofloxacin and its active metabolite ciprofloxacin penetrated AH after subcutaneous administration and reached therapeutic levels for sensitive microorganisms as Staphylococcus intermedius and Escherichia coli with MIC values less than 0.25 mg/mL but concentrations were below the MIC90 for Pseudomonas aeruginosa