Hydrocortisone, single administered into rats 5 hrs before the experiment, did not alter oxidative phosphorylation in liver mitochondria but inhibited the process in spleen. In spleen a decrease in content of SH-groups and of nucleic acids was also observed. An inhibition of oxidative phosphorylation and a decrease in content of SH-groups occurred in mitochondria of spleen and liver tissues after administration of hydrocortisone within 5 days. Together with the phenomenon content of RNA and DNA was decreased in nuclei and homogenates of the tissues studied. The trend to normalization of the phosphorylation process and the increase in content of sulfhydryl groups were found in mitochondria after simultaneous administration of insulin. In this case content of nucleic acids was also increased in nuclei and homogenates of spleen and liver tissues. Possible mechanisms of the normalizing effect of insulin on the energy production and the nucleic acids accumulation in these tissues, impaired by administration of hydrocortisone, are discussed.