To determine whether administration of nitrous oxide, 50% and 70%, could provide analgesia and anxiolysis during venous cannulation in pediatric patients, 165 ASA Physical Status 1 patients scheduled for elective surgery were studied. Children, 3 weeks to 18 yr of age, were randomly assigned either to receive nitrous oxide, 50% or 70% in oxygen, or 100% oxygen via mask or to a group breathing room air, for 3 min prior to and during venous cannulation. A blinded observer using a behavioral scale for rating pain in children performed assessments of behavior and pain before and following venous cannulation. Children who received 50% or 70% nitrous oxide were more likely to be relaxed, 59% and 84%, respectively, and had little evidence of pain. Of those given 100% oxygen or no mask, only 30% and 21%, respectively, were considered relaxed, and 16% and 15% had little evidence of pain during venous cannulation. Side effects were seen in 28% of the group given 70% nitrous oxide and included excitement, dysphoria, nausea, restlessness, and opisthotonic movements. Both 50% and 70% nitrous oxide in oxygen administered to pediatric patients are effective at decreasing the pain and anxiety associated with venous cannulation, but use of the latter is associated with side effects.