The firing rate of the nerves innervating interscapular brown adipose tissue (IBAT) and IBAT and colonic temperatures (TIBAT and TC were monitored in urethane-anaesthetized male Sprague-Dawley rats. These variables were measured for 40 min before (baseline values) and 40 min after a 4 micromoles L-arginine (L-arg) or 400 nmoles nitroprusside (NP) injection in a lateral cerebral ventricle and an intracerebroventricular administration of 500 ng prostaglandin E1 (PGE1). The same variables were monitored in other rats with L-arg or NP or PGE1 administration alone. No drug was injected in control rats. The results show that L-arg or NP injection reduces the increases in firing rate, TIBAT, Tc induced by PGE1. These findings suggest that nitric oxide is important in the control of thermogenic changes during the PGE1 hyperthermia.