A metabolism trial was conducted with four Brahman cattle and four swamp buffaloes fed with Ruzi grass hay (Brachiaria ruziziensis, 0.77% N). There was no significant difference in energy and nitrogen balances, and nutrient digestibilities between cattle and buffaloes. However, fasting heat production was significantly lower in buffaloes. Urinary excretion of allantoin tended to be lower in buffaloes than in cattle although there was no significant difference. Free fatty acid content in the blood serum during the fasting period was lower in buffaloes than in cattle although it was not different when the animals were fed. On the other hand, urea content in the blood serum during the fasting period was higher in buffaloes than in cattle.