STUDY OBJECTIVE: To compare the respiratory effects of tramadol and oxycodone. DESIGN: Placebo-controlled, double-blind randomized study. SETTING: IV Department of Surgery, Helsinki University Central Hospital. PATIENTS: 36 ASA physical status I and II patients undergoing minor surgery with general anesthesia. INTERVENTIONS: The respiratory effects of intravenous (i.v.) tramadol 0.6 mg/kg and oxycodone 0.04 mg/kg were compared after induction of anesthesia with propofol and succinylcholine-facilitated endotracheal intubation. Patients spontaneously breathed halothane in 70% nitrous oxide and oxygen via a nonrebreathing valve. The trial drugs or placebo were given after recovery from neuromuscular block. MEASUREMENTS AND MAIN RESULTS: Inspiratory and expiratory oxygen and end-tidal carbon dioxide concentrations (ETCO2), tidal volume (VT) minute volume of ventilation (VE), and respiratory rate (RR) were recorded by side-stream spirometry with end-tidal halothane of 0.3% for 30 minutes before surgery. Oxycodone caused a significant respiratory depression seen as an increase in the inspiratory-expiratory oxygen difference and ETCO2 and as a decrease in VE and RR. On the contrary, the effect of tramadol were similar to those of placebo. VT was not affected by any study drug. CONCLUSION: Tramadol was not associated with respiratory depression in the present setting.