Catecholamines and some of their metabolites were determined in urine and blood plasma of guinea-pigs before, during and after acclimation to a cold or warm environment. During adaptation to 5 degrees C the amounts of noradrenaline in plasma and 24-h urine samples continuously increased up to 600% compared with values obtained at an ambient temperature of 22 degrees C. Higher levels of dihydroxyphenylglycol and 3-methoxy-4-hydroxyphenylglycol further indicated an increased turnover of noradrenaline during cold adaptation. Acclimation to an ambient temperature of 28 degrees C reduced the peripheral release of noradrenaline in comparison to the release observed at 22 degrees C. Cold-induced increases in metabolic rate and electrical muscle activity both occur at a considerably lower mean body temperature in cold-than in warm-adapted guinea-pigs. The shift of thermoregulatory cold defence reactions to a lower mean body temperature could also be observed in warm-adapted animals after intramuscular infusion of noradrenaline in amounts comparable to those released during cold adaptation. It is concluded that high peripheral sympathetic activity directly or indirectly inhibits noradrenergic neurons in the lower brain stem that modulate the thermoregulatory control system by means of their afferents to the hypothalamus. As a consequence of this peripheral influence the thermoregulatory set point is shifted to a lower mean body temperature.