The sympathetic firing rate of the nerves innervating interscapular brown adipose tissue (IBAT), IBAT and colonic temperatures (TIBAT and TC) were monitored in urethane-anaesthetized male Sprague-Dawley rats. These variables were measured for a period of 40 min before (baseline values) and 40 min after a 2 mg NG-methyl-L-arginine (NMA) injection plus an intracerebroventricular administration of 500 ng prostaglandin E1 (PGE1) into a lateral cerebral ventricle. No drug was injected in control rats. The results show that NMA enhances the increases in firing rate, TIBAT and TC induced by PGE1. These findings indicate that an inhibitor of nitric oxide synthesis, such as NMA, increases the sympathetic and thermogenic responses to injection of PGE1.