The authors examined the effect of the opiate antagonists naloxone and thyrotropin-releasing hormone (TRH) on neurological outcome and the size of areas of cerebral infarction in a rat model of focal cerebral ischemia. The middle cerebral artery (MCA) was permanently occluded in 66 adult Sprague-Dawley rats. The rats were randomly divided into three groups. In 20 Group I rats, TRH in normal saline was administered initially as a 2-mg/kg bolus followed by continuous infusion of 2 mg/kg/hr for 4 hours. In 20 Group II rats, naloxone in normal saline was administered initially as a 2-mg/kg bolus followed by continuous infusion of 2-mg/kg/hr for 4 hours. In 26 Group III rats, physiological saline was administered as an initial 0.5-cc bolus followed by continuous infusion of 0.5 cc/hr for 4 hours. All solutions were given in volumes of 0.5 cc for the bolus and 0.5 cc/hr for continuous infusion, and all infusions were begun within 10 minutes of MCA occlusion. Twenty-four hours after treatment, the rats underwent a careful neurological examination and were then sacrificed immediately. The size of areas of cerebral infarction was evaluated using 2,3,5-triphenyltetrazolium chloride staining techniques. The neurological grade of the rats correlated with the size of infarcted areas among all grades, irrespective of treatment (p less than 0.01). Neither naloxone nor TRH improved neurological function or reduced the size of infarction compared to saline-treated control rats. Treatment with TRH caused a significant increase in mean arterial blood pressure during infusion, but naloxone had no effect. These results suggest that neither TRH nor naloxone are effective in the treatment of acute focal cerebral ischemia.