1. The thermogenic, cardiovascular and metabolic responses to the ingestion of a 30 kJ/kg body-weight test meal were studied in six normal weight, female subjects before and after a 7 d period of underfeeding at 60 kJ/ kg ideal body-weight per d.2. With underfeeding there were decreases in body-weight, plasma insulin and 3,5,3'-triiodothyronine concentrations, resting metabolic rate and respiratory exchange ratio, with increased blood ketone levels. Baseline ‘arterialized’ venous plasma noradrenaline and adrenaline concentrations were not affected by underfeeding.3. Ingestion of the test meal caused similar increases in heart rate and calf blood flow and changes in blood pressure in the fed and underfed states. There was a greater glycaemic response to the test meal in the underfed state compared with the fed state although the rise in plasma insulin concentration was similar and ketogenesis was suppressed. The increases in metabolic rate and plasma noradrenaline concentrations following the test meal were similar in the fed and underfed states.4. Although the period of underfeeding in the present study led to considerable metabolic adaptation, and some alteration in physiological responses to ingestion of a test meal, there was no evidence that there were associated changes in sympathetic nervous system activation.