This paper reports a single-blind study of analgesia in two groups of 14 patients after major abdominal surgery. Both groups received the same dosage scheme of diamorphine, consisting of a 3-mg loading dose followed by a 48-h post-operative infusion of 20 mg in 10 ml of 0.9% NaCl solution. One group (E) received the diamorphine epidurally and the other group (I) received it intravenously. The infusions produced plasma concentrations of morphine in a similar range (3-5 ng ml-1) in both groups. Both groups were encouraged to supplement their analgesia until they were comfortable, using pethidine 50 mg 2 hourly as necessary, and both groups appeared to do so equally successfully. However, patients receiving the epidural infusions received their first pethidine supplements significantly later, and required a significantly smaller amount (about one-third) of supplementary pethidine over the 48 h of the infusion.