Thyrotropin releasing hormore(TRH) administered intracerebroventricularly (i.c.v) in microgram quantities to anesthetized rabbits, produces marked stimulation of colonic motility, transit and fluid accumulation. These effects depend on an intact vagal system. Atropine blocks hypermotility but not the increased intestinal transit and fluid accumulation. The latter effects are antagonized by antiserotonin compounds. In this study, we found that pretreatment of rabbits with naloxone(2.5 mg/kg) or naltrexone(1.0 mg/kg) attenuated or blocked the TRH-induced colonic transit and increase in luminal fluid, but not the hypermotility. In this respect the narcotic antagonist effects resemble those produce by the antiserotonine compounds or opiate agonists.