The experiments were performed on intact rabbits weighing 2.5-3 kg. The animals were treated with penicillin or streptomycin injected intramuscularly in doses of 20 000 units or 17 000 micrograms a day per kg bw for 12 days. The concentrations of lactic and pyruvic acids, cytochrome c, activity of cytochrome c oxidase and thiol spectrum were investigated in the lungs and liver immediately after the antibiotic use in 10 rabbits of every experimental series. To follow the period of persisting of the antibiotic-induced changes the other 10 rabbits were killed 29 days after the last injection of the drugs. It was shown that penicillin induced significant changes in the concentrations of lactic and pyruvic acids in the lung tissue, activated cytochrome c oxidase and decreased the concentrations of free SH and S-S groups immediately after its administration. 29 days after discontinuation of penicillin the activity of cytochrome c oxidase returned to normal, while the other biochemical indices remained changed. Penicillin induced changes in the concentrations of lactic and pyruvic acids and the activity of cytochrome c oxidase in the liver. The concentrations of free masked SH and S-S groups also changed. 29 days after discontinuation of penicillin the enzymatic system of cytochrome c-cytochrome c oxidase alone returned to normal. Like penicillin, streptomycin induced changes in the tissue respiration of the lungs and liver. The character of the streptomycin-induced changes was more pronounced both immediately and 29 days after discontinuation of the drug use.