Pigs were reared from 2 wk of age in either 10 or 35 degrees C and fed ad libitum. At 8 wk of age they were tested for the presence of regulatory nonshivering thermogenesis by administration of norepinephrine (NE) and propranolol. In addition, an electromyogram and carotid temperature, as well as a heat flow and skin temperature from one site, were monitored while the pigs were at ambient temperatures of 10, 15, 20, 25, and 30 degrees C. Cold-reared pigs showed a heightened response to NE in cold compared to thermally neutral environments. This was not observed in warm-reared pigs. Propranolol depressed O2 consumption more in cold-reared than in warm-reared pigs. Pigs reared in the cold also showed a higher intensity of shivering, tissue conductance, and skin temperature than warm-reared littermates. The shivering response of cold-reared pigs was more sensitive to changes in skin temperature than in warm-reared pigs.