BACKGROUND: The combination of propofol and alfentanil for nonrelaxant intubation produces hypotension and bradycardia. N2O, a commonly used inhaled anesthetic, has been known to augment the anesthetic depth in propofol anesthesia. Conversely, N2O was reported to increase the incidences of opioid-induced cough and rigidity. This study was designed to evaluate the effect of additional N2O on propofol requirement and intubating conditions. METHODS: Eighty healthy premedicated female patients were divided into N2O group (n = 40) and non-N2O group (n = 40). In each group, they were randomly assigned to four subgroups according to the propofol dose (1.0-2.5 mg/kg). With or without N2O (FiN2O = 0.5), propofol and 30mug/kg of alfentanil were administered during the induction of anesthesia. Expired concentrations of N2O were measured. Intubation was the attempted and intubating condition was scored (0-6). Incidences of cough and rigidity were also recorded. Dose of propofol for smooth intubation (score > 5) was analyzed and compared between groups. The conditions for smooth intubation were analyzed with variables (expired concentration of N2O, dose of propofol, age, incidences of hypotension, bradycardia, cough and rigidity) by logistic regression. RESULTS: The expired concentration of N2O was 33.0-3.8%. Propofol ED50 for smooth intubation was 1.67 mg/kg (1.26-2.19) in the N2O group and 2.27 mg/kg (1.78-3.47) in the non-N2O group. A smooth intubating condition was correlated well with increased concentrations of N2O and doses of propofol and inversely correlated with incidences of cough and rigidity. However, we failed to prove a significant difference in incidences of hypotension, bradycardia, cough and rigidity between the two groups. CONCLUSIONS: We cannot reduce the propofol requirement for smooth intubation without relaxant by using N2O. The success rate of intubation was increased by additional N2O.