The absorption of morphine 10 mg after intramuscular injection into the gluteal and deltoid muscles was investigated. Ten patients received the injection into the deltoid and 10 were given the injection into the upper outer quadrant of the buttock. Blood was taken at frequent intervals up to 2 hours after the injection for plasma morphine analysis by high performance liquid chromatography. Morphine concentrations were generally higher in the gluteal group although the mean peak concentrations (62.8 ng/ml in the gluteal group and 52.3 ng/ml in the deltoid group) were similar. Peak morphine concentrations varied from 22.5-99.3 ng/ml in the gluteal group and 26.5-84.5 ng/ml in the deltoid group. The area under the concentration-time curve was significantly greater in the gluteal group than in the deltoid group but this difference disappeared when allowance was made for differences in body weight of the two groups. We conclude that the absorption of morphine from the deltoid and gluteal sites is similar.