Plasma concentrations of pethidine following i.m. gluteal injection were measured in surgical patients and volunteers. The mean plasma concentrations tended to be higher in the patients than in the volunteers; this may be a result of a slower initial absorption rate. At least 80% of the dose was absorbed from the injection site over the 6-hr period of the study. Fluctuations in plasma pethidine concentration were observed, probably caused by variations in local and systemic blood flow. This effect was more noticeable in the patient group. In general, the time-course of subjective effects in volunteers could be related to maximum plasma pethidine concentrations. However, patients appeared to be less sensitive to these effects at similar plasma drug concentrations, possibly because of catecholamine-mediated stimulus, suggesting that plasma concentrations may be a poor guide to the clinical response in patient-volunteer comparison.