The effects of naloxone (8 mg) on the pressor and plasma catecholamine response to a standard cold-pressor test have been evaluated in six normal male subjects. Plasma catecholamines were estimated by high performance liquid chromatography coupled to electrochemical detection. Cold stimulation induced significant elevations in plasma noradrenaline and adrenaline to reach mean peak levels 61% and 108% above their respective basal levels (P less than 0.05). Systolic blood pressure increased by 23 +/- 6.5 mmHg (P less than 0.001), and heart rate increased by 7.5 +/- 2.5 beats/min (P less than 0.001). Naloxone pretreatment significantly enhanced the plasma adrenaline response to the cold stimulus by 98% (P less than 0.01) with concomitant changes in peak systolic blood pressure (peak increment 31 +/- 6 mmHg) and pulse rate (12.5 +/- 3.5 beats/min) responses (both P less than 0.05). The mean plasma noradrenaline response to cold also increased after naloxone, but this failed to achieve significance. Endogenous opioids are likely to be involved in the sympathoadrenal response to a mild acute stress in man.