Although many deaths occur annually from heroin intoxication, the presence of heroin has not been reported in postmortem tissues. Recognizing heroin's susceptibility to rapid chemical and metabolic hydrolysis, extraction procedures were developed for the efficient recovery of heroin, 6-acetylmorphine, and morphine from postmortem tissue utilizing solid-phase extraction coupled with gas chromatography/mass spectrometry. From heroin-related deaths, 21 sets of blood and urine specimens were collected. The mode of death in these cases was categorized as rapid, delayed, or undetermined. Compared with delayed deaths, rapid deaths were characterized by the following trends: higher mean concentrations of 6-acetylmorphine, free morphine, and total opiates in blood; a higher ratio of free morphine concentrations to total opiate concentrations in blood; lower mean concentrations of 6-acetylmorphine and morphine in urine; greater likelihood of 6-acetylmorphine detection in blood; and lesser likelihood of heroin detection in urine. The study also included analysis of multiple tissue specimens from two subjects who died of heroin intoxication. Heroin was identified in urine and injection-site tissue. Concentrations of 6-acetylmorphine in cerebrospinal fluid, spleen, and brain were substantially higher than in blood, liver, lung, and kidney. All specimens were positive for morphine. Heroin metabolites were detected in hair specimens. The identification of heroin and 6-acetylmorphine in biological tissues effectively established the presence of heroin in cases of acute narcotic intoxication. These studies demonstrated that measurement of heroin and its metabolites provides useful information for the differential diagnosis of heroin-related deaths.