BACKGROUND: Total intravenous anesthesia with propofol can cause respiratory depression and apnea especially during induction of anesthesia. To study the possibility of reversal of respiratory depression during anesthesia with propofol, pretreated with nabuphine or not, the respiratory effects of doxapram to spontaneously ventilating patients were investigated. METHODS: Patients were divided into 4 groups - saline-propofol-saline group (SPS), saline-propofol- doxapram group (SPD), nalbuphine-propofol-saline group (NPS), and nalbuphine-propofol-doxapram group (NPD). After saline or nalbuphine pretreatment, anesthesia was induced with propofol and then doxapram or saline was intravenously injected. Apneic time interval, blood pressure, heart rate, respiratory rate, minute ventilation, end tidal CO2 partial pressure and oxygen saturation were measured in every minutes during induction of anesthesia. Percent changes of each values were compared. RESULTS: There is no differences in apneic time intervals in each groups. The percent change of first minute ventilation in SPD group after doxapram injection unchanged significantly compared with those depressions of SPS, NPS and NPD group (p<0.05). Respiratory rates increased in SPD and SPS groups after laryngeal mask insertion. There is no differences in minute ventilation, respiratory rate and end-tidal CO2 concentration between nalbuphine pretreated groups regardless of doxapram injection. CONCLUSIONS: Doxapram has effect in increasing minute ventilation after propofol induction within first few minutes, but it cannot reverse respiratory depression during propofol induction pretreated with nalbuphine.