Abstract The effects of the species specific hypoglycaemic agent cyclopropanecarboxylic acid and its carnitine ester on certain synthetic and oxidative mitochondrial processes in rat and guinea-pig liver were examined. No major species differences were found in the following responses. Cyclopropanecarboxylate (0·2 mM) inhibited oxygen uptake and 14CO 2 evolution from [1- 14C]acetate and [1- 14C]palmitate but not from [1- 14C]laurate, [1- 14C]octanoate or [1- 14C]butyrate. Palmitoyl carnitine utilization was not significantly affected but palmitoyl CoA levels were decreased. The oxidation of tricarboxylic acid cycle intermediates was not significantly inhibited. Acetoacetate synthesis from fatty acids was inhibited possibly at the stage of HMG CoA formation. Cyclopropanecarboxyl carnitine had little effect on fatty acid oxidation and ketogenesis. Possible mechanisms for the inhibitory effects of Cyclopropanecarboxylate are discussed.