In fed and starved carp, Cyprinus carpio, ketone body metabolism and those metabolic and endocrine factors that are known to induce ketogenesis in starving mammals were investigated. Acetoacetate was detected in plasma and liver of both fed and starved carp. We could not detect 3-hydroxybutyrate, neither by ¹H-NMR spectroscopy nor by spectrophotometric assay, in spite of low activities of hepatic 3-hydroxybutyrate dehydrogenase. Starvation of carp did not create metabolic conditions that would favor ketone body synthesis: Mobilization and hepatic catabolism of fatty acids were only moderately enhanced, the rate of gluconeogenesis was not elevated, and glucagon levels as well as the glucagon/insulin-ratio remained stable or declined. Therefore, the discrepancy in the effect of food deprivation on mammalian and teleostean ketogenesis appears to be caused not by the absence of the ketogenic pathway from teleosts but by major differences between mammals and fish in their metabolic response to starvation.