The aim of the study was to investigate the effects of oral application of enrofloxacin (Baytril®) on the indices of ruminal functions of adult cattle (in vivo). Five clinically healthy Simmental cows, 5-7 years old were used as a material of study. The study was designed to 3 phases: Phase A (2 days before oral application of Baytril®), phase B (in which Baytril® was given orally in a dose of 5 mg/kg body weight twice daily for a period of 5 successive days and phase C (5 days after withholding of Baytril® and lasted for another 2 days). Ruminal fluid and blood samples were taken twice daily and examined during the course of study. The study revealed a negative temporary influences of orally applied Baytril on the activity of ruminal flora and fauna, which were represented by weakness of rumen motoric activity in 2 cows, three days after application of drug, increased time of sedimentation activity test (SAT), increased time of methylene blue reduction test, reduced levels of acetic and propionic acids associated with slight increase in ruminal ammonia and chloride. Microscopical examination of ruminal fluid showed decreased density, viability and activity of protozoa. Blood examination revealed significant decrease in plasma ammonia. Ruminal flora and fauna resumed its biological activities 5-7 days after withholding of the drug. It could be concluded that in vivo application of Baytril 5 mg/kg body weight over a 5 successive days revealed a minimal detrimental effect on rumen fermentation which returned back to its normal pattern one week after withholding of the preparation and the drug could be used orally when indicated.