Prostaglandin E2 at concentrations of 5.6 mM and 56 mM, in presence of EDTA 2.2 mM, did not affect the patterns of respiration and coupling, registered by means of polarographic procedure, in isolated rat liver mitochondria. Activation of phosphorylating and uncoupled respiration of mitochondria was noted in incubation mixtures containing EDTA 2.2 microM, Ca2+ 0.2 microM, prostaglandin E2 5.6 microM. In absence of EDTA prostaglandin E2, at concentrations of 5.6 microM and 56 microM, caused uncoupling of the oxidative phosphorylation and in presence of Ca2+--it caused uncoupling and inhibition of mitochondrial respiration, decrease in the rate of phosphorylation and in the ATPase reaction was well as in the efficiency of mitochondrial proton pump, connected with Ca2+ transport. At concentrations 2.8 microM, 5.6 microM, 14 microM protaglandin E2 increased the proton conductivity of the mitochondrial membrane in presence of K+ and valinomycine. The data obtained suggest that the effect of prostaglandin E2 on the energy functions of mitochondria depends on presence of Ca2+ in the incubation mixture.