The addition of phentolamine enhances oxygen uptake of rat heart homogenates respiring in the presence of added succinate, but evokes an inhibitory effect with pyruvate as the substrate. These effects are not observed when phentolamine is replaced by tolazoline. Oxidation of α-ketoglutarate is not affected by either drug nor is the activity of a purified malic dehydrogenase. The addition of oxaloacetate to homogenates inhibits oxygen uptake and the inhibitory effect is not abolished by the prior addition of phentolamine. The total α-keto acid content of homogenates respiring in the presence of succinate was found to be somewhat higher than in those homogenates to which both succinate and phentolamine were added at the beginning of the experiments.