A study is presented on the effect of the β-adrenoceptor agonist isoproterenol on mitochondrial oxygen metabolism in fibroblast and heart myoblast cultures. Isoproterenol treatment of serum-limited fibroblasts and proliferating myoblasts results in the promotion of mitochondrial complex I activity and decrease of the cellular level of reactive oxygen species. These effects of isoproterenol are associated with cAMP-dependent phosphorylation of complex I subunit(s). Addition of okadaic acid, inhibitor of protein phosphatase(s), reverses the decline of complex I activity in serum-limited fibroblast cultures and activates the complex in proliferating myoblast cultures. The effects of isoproterenol on complex I activity and reactive oxygen species balance can contribute to the therapeutic effect of the drug.