We have examined the relationship between the changes in resting metabolic rate (RMR) and those in hepatic metabolism induced by hyperthyroidism and fasting for 24 h. We found that hyperthyroidism induced a significant increase in RMR, while fasting for 24 h reduced RMR in euthyroid but not in hyperthyroid rats. We have also measured oxygen consumption in isolated hepatocytes from euthyroid and hyperthyroid rats, fed or fasted for 24 h. Hyperthyroidism induced an increase in oxygen consumption in rat liver cells; fasting for 24 h increased respiratory rates in isolated liver cells from euthyroid but not from hyperthyroid rats. The findings showed that hyperthyroidism and fasting for 24 h have opposite effects on RMR but similar effects on hepatic metabolism. The results also indicated that the increase in RMR found in hyperthyroid rats is partly due to an increase in hepatic metabolism, while no correlation exists between variations in resting and hepatic metabolism induced by 24-h fasting.