A pilot study of prehospital analgesia with 50% nitrous oxide and 50% oxygen was undertaken in patients experiencing severe pain from various sources. Under the supervision of an ambulance attendant N2O/O2 was administered through a face mask held by the patient and connected to a portable regulator/tank unit. Two types of units were evaluated -- Entonox (with premixed N2O and O2) and Nitronox (with separate cylinders of N2O and O2, the gases being mixed at the time of administration). Of the 72 patients 69 obtained worthwhile analgesia (marked or partial relief of pain) during treatment in the field or in the ambulance. There were no serious side effects, and those that did occur reflected N2O's expected action (e.g., giddiness). N2O/O2 is thus considered a safe and effective analgesic, suitable for use by ambulance personnel.