The responses of circulating catecholamines, TSH, thyroid hormones and prolactin to 30 min of acute cold exposure (4 degrees C) were measured in eight normal volunteers over a 2 h period. There was a rise in circulating noradrenaline, TSH, T4 and T3 levels and a fall in circulating prolactin in the subjects studied, but no change in circulating adrenaline levels nor any alteration in the T4/T3 ratio. The thyroid axis of normal individuals can respond rapidly to acute cold exposure. In addition, the increased plasma noradrenaline levels accompanied by unaltered adrenaline levels suggest that the stimulus exerted by cold does not evoke a generalised stress response, but rather that the sympathetic nervous system is selectively stimulated.