Aim. To compare the changes of intestinal microbial composition of two models of dysbiosis induced by ceftriaxone (Cf) or the mix of ampicillin (Amp) and metronidazole (Met). Methods. The object of the study was the changes of fecal and mucosa-associated microbiota of colon and small intestine of Wistar male rats (m=170-200g, n=19). Cf was injected once a day for 14 days at a dose of 300 mg/ kg i.m. The mix of Amp (75 mg/kg) and Met (50 mg/kg) was injected once a day for 3 days per os. Animals were removed from the experiment in 1 day and 56 th days after treatment withdrawal. The microbiological analysis of the fecal (CFU/g) and mucosa-associated (CFU/cm 2) biotopes of the rats were carried out bacteriologically by sowing the dilutions into the selective diagnostic media for Bifidobacterium, Lactobacillus, Clostridium, E. coli, opportunistic enterobacteria, Staphylococcus and hemolytic bacteria. Results. It was shown that the changes of fecal microbiota after Cf withdrawal progressed with time and continued until the 56th day of observation. Cf administration induced 2 folds decrease the number of colon mucosa-associated anaerobic bacteria Bifidobacterium and Lactobacillus. Moreover, there was over growth of bacteria in the small intestine in 56 days. The mix of Amp/Met induced dysbiosis on the 1st day after the mix withdrawal. The normalization of the colon microbial composition was observed in 56 days.. The mix of Amp/Met increased only quantity of opportunistic enterobacteria and lactose-positive E.coli in mucosa-associated microbiota of small intestine in 56 days after the mix withdrawal. Conclusions. Injecting of the mix of Amp/Met to rats is more adequate model for modelling acute dysbiosis. Cf use induced longterm profound changes in microbiota composition and might be suitable to model chronic dysbiosis.