Twenty healthy male volunteers, dressed in shorts, stayed for 30 min in a room with an ambient temperature of 28 degrees C followed by a stay in a room with a temperature of 10 degrees C for 120 min. The mean skin temperature fell rapidly during the first minutes in the cold but the rectal temperature began to fall as late as at 60 min (0.1 degree C) and was 0.4 degrees C lower at the end of the cold exposure than before it. The metabolic rate, and the systolic and diastolic blood pressures, increased, and the pulse rate fell, in the cold. Serum samples were taken before moving to the cold (10 degrees C) room and after the 2-h stay and assayed for 11 hormones. There were no significant changes in the serum concentration of adrenalin, T3, T4, testosterone, TSH or LH. The serum level of noradrenaline increased from 4.5 to 6.3 nmol l-1 (P less than 0.01) and those of cortisol, GH and prolactin fell by 20, 87 and 48% (all P less than 0.01). The total serum proteins increased by 11% and free fatty acids by 28% (P less than 0.01). Our results show that the short-term exposure of adult man to low ambient temperature does not have any effect on the pituitary-thyroid and pituitary-testis axes and adrenal medulla. The increase of noradrenaline is probably due to general activation of the sympathetic nerves at low temperatures. The decreases in the serum levels of GH and prolactin reflect a true decrease in their secretions and may be mediated by inhibitory hypothalamic mechanisms.