Plasma morphine concentrations were measured by a radioimmunoassay technique following the intramuscular administration of morphine 0.2 mg/kg body weight as a pre-operative medication. The highest plasma concentrations were obtained about one hour after the administration of the drug and there was a sevenfold variation in these concentrations between individuals. For 24 hr following a single intramuscular injection, there were measurable quantities of morphine or its metabolites in the plasma. In healthy patients who were undergoing a therapeutic abortion, there was no correlation between pain relief and the plasma concentration of morphine