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INTRODUCTION: Postoperative analgesia is insufficiently done due, among others, to the undesirable effects of analgesic agents. OBJECTIVE: The aim of this study was to analyze the effects of the simultaneous administration of opiates (meperidine) and AINES (lysine acetylsalicylate, ASL). MATERIAL AND METHODS: We studied 160 patients during the immediate postoperative phase. All of them underwent programmed surgery with the same general anesthetic technique. Patients were allocated into 8 groups of treatment: A) ASL 900 mg and 1.800 mg/8 h, B) ASL 900 mg and 3.600 mg/8 h, C) ASL 900 mg and meperidine 100 mg/8 h, D) ASL 900 mg and 1.800 mg/8 h together with meperidine 100 mg/8 h, E) meperidine 50 mg and ASL 1.800 mg/8 h, F) meperidine 50 mg and ASL 3.600 mg/8 h, G) meperidine 50 mg and 100 mg/8 h, and H) meperidine 50 mg and 100 mg/8 h together with ASL 1.800 mg/8 h. The effects of analgesic agents were evaluated on the basis of patient's appreciation of the degree of pain and relief and on the basis of an observer who did not know the therapeutic regime administered. Results were compared according to the analysis of variance in a graded factorial design. A p value less than 0.05 was considered significant. RESULTS: The degree of pain was significantly lower in groups C, D, G and H (specially in G and H) than in the remaining groups, but there were no significant differences between them. The lowest pain relief was observed in groups A, B, E and F. The highest attenuation of pain was achieved in groups G and H. The highest attenuation of pain was achieved in groups G and H. The observer considered that the two latter groups were those with the highest pain relief, followed by groups C and D. The remaining patients failed to show appreciable improvement. Nausea and vomiting only occurred in some patients after administration of a bolus of meperidine. There were no other secondary effects. CONCLUSIONS: The best degree of postoperative analgesia is achieved after administration of continuous infusion of meperidine 100 mg/8 h. Simultaneous infusion of ASL 1.800 mg/8 h did not improve the analgesia obtained with a bolus of 900 mg of ASL nor with a bolus of 50 mg of meperidine. Secondary effects were only nausea and vomiting and coincided with the administration of a bolus of meperidine.