The purpose of the study was to determine the effect of composition of various breathing mixes on physical capacity of rats swimming in hyperbaric conditions. The said effect was determined on the basis of results of a swim test performed in a pressure chamber. The study was performed with the use of atmospheric air, a mixture of nitrogen and oxygen (N2/O2) at a ratio of 89.5/10 and 92/7.5, as well as a mixture of argon and oxygen at a ratio of 79.5/20 (Ar/O2). The tests were conducted at a pressure range between 0-4 atm. The results suggest that the physical capacity of the tested animals decreased along with pressure increase regardless of the breathing mix used. Due to the fact that the burdening of rats with physical effort in hyperbaric conditions intensifies the adverse effects of components of breathing mixes on their performance, it seems appropriate to continue the study of physiological responses to breathing mixtures of various compositions in human body subjected to physical effort while under water.