The oxidation of amino acids, Krebs cycle intermediates, fatty acids and ketone bodies was examined in a well-coupled mitochondria1 preparation from the liver of the little skate, Raja erinaceu Glutamate was the preferred amino acid on the basis of both the respiratory control ratio (RCR; 7.97 0.60 SE) and the state 3 rate (12.50 (+ 1.16 SE) nmoles 0 min (mg protein-'). Other amino acids, including sarcosine and 0-alanine, were oxidized at less than one third the rate of glutamate with RCRs less than 4. Oxidation of ornithine and arginine was not detected. Of the Krebs cycle intermediates tested, isocitrate had the best RCR and state 3 rate. Malate enhanced the oxidation of tricarboxylic acids (isocitrate and citrate) and a- ketoglutarate, probably by stimulation of transport into the mitochondria. Oxaloacetate and succinyl CoA were not oxidized at detectable rates. Pyruvate oxidation was low, even at high concentrations (10 mM). Fatty acids (as carnitine esters) were oxidized at high rates, especially hexanoyl (C-6), octanoyl (C-โฌ9, decanoyl (C-101, lauroyl (C-121, and myristoyl (C-14) carnitine. Acetoacetate oxidation was not detected, but high rates of oxygen consumption were observed in response to 1 mM 0-hydroxybutyrate. Oxidation of this ketone body was malonate-sensitive, although 14C02 production from 3 (14C)-p-hydroxybutyrate was minimal, indicating that ketone body carbon was not entering the Krebs cycle. The malonate sensitivity of the oxidation process was due to direct inhibition of 0-hydroxybutyrate dehydrogenase (150 for malonate = 0.558 (k 0.031 SE) mM). The liver of this skate appears to be well suited to utilize fatty acids rather than ketone bodies or carbohydrate.