Treatment with alpha adrenergic receptor blocking agents has been used clinically for many years to obviate peripheral vasoconstriction in patients suffering from circulatory or thermoregulatory distress. Various reports indicating that administration of these agents may be followed by an increased plasma concentration of catecholamines, and subsequently an increased oxygen demand, led to 29 investigations on humans who were placed in a horizontal position in a thermoneutral environment and given 25 mg chlorpromazine intravenously, dissolved in 500 cc low molecular weight dextran. Blood pressure and heart rate remained unchanged during the infusions and significant changes in plasma noradrenaline (average fall: 4.42 nmol X 1(-1), range: -31.9- + 5.1) and in plasma adrenaline (average fall: 1.06 nmol X 1(-1), range: -16.3- + 5.11) were observed (P less than 0.01). The results indicate that as long as any baro-reflex activation of the sympathetic nervous system and acceleration of the nonshivering thermogenesis is avoided, the recommended treatment with alpha adrenergic receptor blocking agents can be performed without any increase in plasma concentration of catecholamines.