Activation of either the A₁ adenosine receptor (A₁R) or A₃R elicits delayed cardioprotection against infarction, ischemia, and hypoxia. Mitochondrial contribution to the progression of cardiomyocyte injury is well-known; however, the protective effects of adenosine receptor activation in cardiac cells with a respiratory chain deficiency are poorly elucidated. The aim of our study was to further define the role of A₁R and A₃R activation on functional tolerance after inhibiting the terminal link of the mitochondrial respiratory chain with sodium azide, in a state of normoxia or hypoxia, in comparison to the effects of the mitochondrial K_ATP channel opener diazoxide. Treatment with 10 mM sodium azide for 2 h in normoxia caused a considerable decrease in the total level of ATP; however, activation of adenosine receptors significantly attenuated this decrease. Diazoxide (100 µM) was less effective in protection. During treatment of cultured cardiomyocytes with hypoxia in the presence of 1 mM sodium azide, the A₁R agonist CCPA was ineffective but A₃R agonist Cl-IB-MECA attenuated the decrease in ATP level and prevented cell injury. Cl-IB-MECA delayed the dissipation in the mitochondrial membrane potential during hypoxia in cells impaired in the mitochondrial respiratory chain. In cells with elevated [Ca²⁺]_i following hypoxia and treatment with NaN₃ or after application of high doses of NaN₃, Cl-IB-MECA immediately decreased the [Ca²⁺]_i toward the diastolic control level. The A₁ receptor agonist was ineffective. This may be especially important for the development of effective pharmacological agents, when mitochondrial dysfunction is a leading factor in the pathophysiological cascade of heart disease.