In heroin overdose deaths the blood morphine concentration varies substantially. To explore possible pharmacokinetic explanations for variable sensitivity to opiate toxicity we studied mortality and drug concentrations in male Sprague-Dawley rats. Groups of rats were injected intravenously (i.v.) with heroin, 21.5 mg/kg, or morphine, 223 mg/kg, causing a 60-80% mortality among drug-naïve rats. Additional groups of rats were pre-treated with morphine for 14 days, with or without 1 week of subsequent abstinence. Brain, lung and blood samples were analyzed for 6-acetylmorphine, morphine, morphine-3-glucuronide and morphine-6-glucuronide. i.v. morphine administration to drug-naïve rats resulted in both rapid and delayed deaths. The brain morphine concentration conformed to an exponential elimination curve in all samples, ruling out accumulation of morphine as an explanation for delayed deaths. This study found no support for formation of toxic concentration of morphine-6-glucuronide. Spontaneous death among both heroin and morphine rats occurred at fairly uniform brain morphine concentrations. Morphine pre-treatment significantly reduced mortality upon i.v. morphine injection, but the protective effect was less evident upon i.v. heroin challenge. The morphine pre-treatment still afforded some protection after 1 week of abstinence among rats receiving i.v. morphine, whereas rats given i.v. heroin showed similar death rate as drug-naïve rats.