To investigate the time of administration and concentration of inhaled nitrous oxide (N 2 O) needed to reduce the pain associated with intravenous (IV) cannulation in children.Prospective, randomized study.Operating room of a children's hospital.73 ASA physical status I and II children, aged 6-15 years, scheduled for elective day or non-day surgery.Children were randomly allocated to one of 4 groups prior to IV insertion of a 24-gauge catheter in the dorsum of the hand: Group 1 (n = 18): 50% N 2 O in O 2 for three minutes; Group 2 (n = 18): 50% N 2 O in O 2 for 5 minutes; Group 3 (n = 18): 70% N 2 O in O 2 for three minutes; or Group 4 (n = 19): 70% N 2 O in O 2 for 5 minutes.Just after the venous cannulation, degree of pain was assessed by examining the faces of the patient by the parent and an operating room nurse.Pain scores obtained from parents of children in Groups 3 and 4 were significantly lower than from those in Groups 1 and 2. Pain scores from the nurse in Group 3 was significantly lower than those in Group 1. However, there was no significant difference in pain score between Group 3 and Group 4. Frequency of side effects was similar among the 4 groups.Seventy percent N 2 O in O 2 given for three minutes was effective for reducing venipuncture pain in children.