Nine patients undergoing cardiopulmonary bypass surgery were given either 2 mg diamorphine or 2.5 mg morphine by intrathecal injection. Spinal fluid (sf) samples were collected over 25 min and drug concentrations measured by HPLC. Concentrations in sf were about 4000 times as great as after 1 mg/kg IV morphine. The kinetic properties of morphine and heroin in sf differed; diamorphine was removed from sf much more rapidly than morphine. Lipophilic opiates may be safer for intrathecal use because of the shorter life of substantial drug concentrations in the mobile sf phase.