The suggestion that the ketonaemic response to fasting may be altered in the tumour-bearing state was investigated by studying the metabolism of fasted C57/BL6j mice bearing transplanted B16 melanomas. Ketone body (D-3-hydroxybutyrate and acetoacetate) concentrations in the blood of the tumour-bearing mice were significantly increased after a 24 h fast compared to control mice with identical dietary histories. Hepatic glycogen levels were lower at the start of the fasting period in the tumour-bearing mice as were the fat stores. The loss of adipose tissue during the fasting period was greater in the tumour-bearing mice. After 48 h of fasting, the ketonaemia was significantly lower in the tumour-bearing mice compared to the appropriate controls. Two distinct metabolic states are indicated in these fasted tumour-bearing mice, one characterised by accelerated ketonaemia, and a later, near terminal stage, where fat deposits are markedly depleted and ketonaemia is decreased.