In 38 patients buprenorphine, meperidine and pentazocine were given in a single dose for postoperative pain relief 20 hours after the end of anaesthesia. Measuring the parameters of the high- and low-pressure system as well as the metabolism the authors found that the effects of these analgetic medicaments, intravenously injected were significantly influenced by fentanyl, halothane or diazepam, given under the course of operation. Especially buprenorphine, injected after epidural anaesthesia in combination with diazepam sedation, proved to have a rather negative effect, because it caused a strong depression of respiration and circulation. On the other hand buprenorphine had, given after neuroleptanaesthesia, a neutralizing - and pentazocine and pethidine in combination with neuroleptanaesthesia a stimulating influence on the circulation. After halothane-anaesthesia the effect of the analegtics on the cardiovascular system was, when buprenorphine was given, depressing and when pentazocine was given indifferent. Similar reactions, but more pronounced, could be seen in the epidural group. With certain reservations, caused by the preliminary character of this study, the following conclusions can be drawn for the anaesthetic practice: 1 Choosing analgetic drugs for postoperative pain relief, the anaesthesist has to be aware of the interactions, possibly resulting from the medicaments, given during anaesthesia. 2. The number of medicaments, given during anaesthesia, should be kept small, considering the eventual interactions and the unintentional secondary effects.