The thermogenic responses to noradrenaline have been studied during food restriction in conscious rats. Acute starvation for 16-18 h reduced the body weight by 3% but not the resting metabolic rate or the increase in metabolic rate induced by noradrenaline, compared to control rats. Sub-acute underfeeding (eight days) with 50% of the control daily intake decreased body weight by 15% and was associated with a significant (P less than 0.05) reduction in resting metabolic rate and a significant (P less than 0.01) suppression of the thermogenic response to noradrenaline. Thus, food restriction reduces the thermogenic responses to noradrenaline, but only after a certain duration of restricted intake.