Ten hours after the 5-day space flight on Cosmos-1514 rats were examined for oxidative phosphorylation in mitochondria isolated from the posterior femoral muscles as well as for Krebs cycle enzymes and glycolysis in the mitochondrial and cytoplasmic fractions of the muscles. The mitochondrial respiration rate in various metabolic states was similar in flight rats and vivarium controls. After flight calculated parameters of energy efficacy of respiration as well as activity of malate dehydrogenase, isocitrate dehydrogenase and total lactate dehydrogenase remained unchanged. Unlike the flight rats, the synchronous controls showed signs of the stress-reaction: uncoupling of oxidative phosphorylation and oxalacetate inhibition of succinate dehydrogenase. Comparison of these findings with those from prolonged space flights indicates that inhibition of oxidative metabolism and glycolysis in mixed muscles which was demonstrated in the 20-day space flight does not develop immediately after launch but occurs within the time interval between mission days 6 and 18.