SummaryPharmacokinetic modelling of estimated central nervous system concentrations was used to devise the optimal mixture of morphine and alfentanil for the treatment of postoperative pain. Modelling revealed that an intravenous opioid pain protocol using an alfentanil2morphine mixture in the proportions 0.75 : 10 mg would provide a profile of analgesia of rapid onset, yet slow offset. The regimen was evaluated in 58 patients in the recovery ward who were randomly allocated to receive analgesia using pain protocols with either morphine or the mixture. Groups were well matched for age, weight and initial pain scores. The mean (SD) time to patient comfort was 27.6 (20.2) min for the mixture and 41.2 (18.6) min for morphine (p 0.01). Multiple regression analysis revealed that that initial pain score (p 0.009) and drug group (p 0.02), but not age, weight or gender were independent predictors of the time to comfort. Drug group was not a significant predictor of adverse effects. Postoperative pain and current dose regimens Despite the best efforts of anaesthetists, many patients still experience moderate to severe pain in the recovery room. This is frequently managed using intravenous opioids titrated to analgesic effect, and nurse-administered`pain protocols' involving repeated intravenous bolus doses of opioid given at fixed`lock-out periods' can be very effective in this setting [1,2].However, commonly used opioids, such as morphine and pethidine, have a relatively slow onset to peak effect even after intravenous bolus dose administration. When repeated doses are given during titration regimens, thè lock-out period' between doses must allow for at least some of the effect of each dose to be seen before another is given. This can result in considerable delays before adequate analgesia is achieved. Increasing the frequency of administration of boluses may speed the onset of analgesia, but will also increase the risk of accumulation [3] and adverse effects such as respiratory depression. Choosing an opioid with a more rapid onset, such as alfentanil, may provide more rapid onset of analgesia, but will result in a brief duration of effect.The aim of this study was therefore to explore the feasibility of a mixture of opioids to produce the features of rapid onset, yet slow offset.Kinetic and dynamic basis of a mixture of alfentanil and morphine The concepts used here arose from a recent review of the opioid pharmacokinetic literature by the authors [3]. A data set of kinetic parameters was compiled from the literature (with various assumptions) and used to simulate the time-course of blood and estimated central nervous system (CNS) concentrations of opioids in various settings. It was noted that, after intravenous bolus administration, the estimated CNS concentrations of opioids were delayed relative to their blood concentrations to different extents depending on their rate of q 2001 Blackwell Science Ltd 739