The influence of vitamin B₁₂ and glucose cycloacetoacetate hydrolysate on oxidative phosphorylation in liver mitochondria of rats fed an atherogenic diet has been studied. Feeding of the atherogenic diet containing high content of cholesterol to animals caused characteristic alterations in the P/O ratios of liver mitochondria. With citrate and pyruvate as substrates, the P/O ratios of the liver mitochondria in the atherogenic diet-fed group are found to be much lower as compared to the normal group. Subcutaneous administration of vitamin B₁₂ or GCAh to atherogenic diet-fed rats resulted in an appreciable increase in the P/O ratios.