Because morphine is often found in the postmortem (PM) blood samples of patients treated with or abusing the drug, its causation in death has to be considered. The possibility of PM redistribution of the opioid drug has not been studied previously. We treated adult Wistar rats with 4 mg/kg of morphine i.m. and measured cardiac cavity levels at the time of death, by killing (2 h postdose) 24 and 96 h PM. Morphine concentrations rose from 41.4 +/- 13.2 ng/ml at death to 111.9 +/- 66.6 ng/ml at 24 h (p = 0.00036), with no additional increase at 96 h (98.7 +/- 21.7 ng/ml). After death there was accumulation of endogenous substances cross-reacting with the morphine radioimmunoassay. Although these attribute only 3% to total accumulation of morphine, they may complicate interpretation of morphine "readings" in patients who had not received the drug. Our study suggests that after death there is substantial redistribution of morphine. Elevated PM levels cannot necessarily by interpreted as representing antemortem concentrations.