The firing rate of the nerves innervating interscapular brown adipose tissue (IBAT), IBAT and colonic temperatures (TIBAT and TC), and O2 consumption were monitored in urethan-anesthetized male Sprague-Dawley rats. These variables were measured for 40 min before (baseline values) and 40 min after a neostigmine (5 x 10(-7) mol) or saline injection in the hippocampus. The blood level of 3,5,3'-triiodothyronine and L-thyroxine (T3 and T4) and the 5'-deiodinating activity of IBAT, liver, and kidneys were determined in other rats with neostigmine or saline injection. The results showed that neostigmine injection increased firing rate, TIBAT, TC, O2 consumption, blood level of T3, and 5'-deiodinating activity of IBAT. No change was found in the T4 level and in 5'-deiodinating activity of the liver and kidneys. These findings suggest that neostigmine injection in the hippocampus increases heat production by stimulating sympathetic nerves to IBAT and by elevating the blood level of T3.