The thermogenic, cardiovascular and metabolic responses to a 30 kJ/kg body-weight test meal were studied in eight normal-weight, healthy female subjects after a 6 or 48 h fast. There was no significant change in metabolic rate following the 48 h fast, but plasma glucose, insulin, noradrenaline and respiratory exchange ratio were all reduced, and plasma beta-hydroxybutyrate was increased. Forearm blood flow was increased, with reduction in diastolic blood pressure. After the 48 h fast, there was a reduction in the metabolic rate response 40-90 min after food (control + 0.54 (SE 0.05), 48 h fast + 0.27 (SE 0.12) kJ/min, P less than 0.01), and in forearm blood flow and diastolic blood pressure responses, but increases in heart rate, blood glucose and plasma insulin responses to the ingestion of the test meal. There was no significant relationship between plasma catecholamine concentration and food ingestion or metabolic rate. Fasting induced considerable adaptation in these subjects and altered some of the physiological responses to food ingestion.