The hypothermic stress of immersion in cold water stimulates release of norepinephrine from the sympathetic nervous system. The speed and pattern of this response was studied in six healthy men by serial measurements of plasma norepinephrine concentrations before, during, and after 60 min of immersion in 10 degrees C water. After immersion for 2 min, the mean norepinephrine concentration was increased from 359+/-32 (basal) to 642+/-138 pg/ml and rose gradually to a maximum of 1.171+/-226 pg/ml after 45 min of immersion. Metabolic rate increased approximately threefold during the immersion period. After rewarming in warm water (40 degrees C), the subjects showed a transient peak in plasma norepinephrine followed by a rapid decrease to basal levels after 30 min. The fall in plasma norepinephrine after approximately 8 min of rewarming occurred despite persistent depression of the core temperature and coincided with a sudden decrease in metabolic rate and cessation of body shivering. These results suggest that the sympathetic nervous response to cold can be activated or suppressed very quickly and is dependent on the skin temperature.