A nimals with lateral hypothalamic lesions lost significantly more weight in the 18 h following this lesion than did sham-operated animals or rats with cerebral cortical lesions deprived of food for the same time period. In the acutely fasted sham-operated animals the turnover of norepinephrine in interscapular brown adipose tissue, heart, and pancreas was slowed but in fasted rats with lateral hypothalamic lesions norepinephrine turnover rates were three- to ninefold faster in all three organs. Exposure to the cold (4 degrees C) significantly increased norepinephrine turnover in the interscapular brown adipose tissue, heart, and pancreas of fasted sham-operated rats, but did not further increase the rate of turnover in lateral hypothalamic-lesioned rats. Rats with lesions in the cerebral cortex responded in a fashion similar to that of the sham-operated animals. Gastric erosions and microhemorrhagic gastric mucosa were observed in five of six acutely fasted rats with lateral hypothalamic lesions whereas all sham-operated rats had a normal appearance of the stomach lining. Animals with lateral hypothalamic lesions made 3 wk earlier also showed an increased rate of norepinephrine turnover in the interscapular brown adipose tissue, heart, and pancreas following an 18 h fast. Rats with bilateral lesions in the paraventricular region of the hypothalamus, however, responded similarly to sham-operated animals with a reduction in the turnover in norepinephrine with fasting and an increase in norepinephrine turnover rate after cold exposure even with fasting. These data suggest that lateral hypothalamic lesions produce an acute increase in turnover of norepinephrine, and that this increased turnover persists for up to 3 wk.