A clinical trial of a 50:50 mixture of nitrous oxide and oxygen for pain relief was carried out to determine the feasibility of its use in a field setting and the side-effects produced by this sedative/analgesic. The gas mixture was delivered from a single-tank system using a demand-valve apparatus which was triggered by the patient's inspiratory effort. This "patient-controlled" sedation/analgesia was provided to 1243 patients over a period of 18 months. Of the 1201 patients evaluated, 20.6% reported minor side-effects consisting of nausea or vomiting (5.7%), dizziness or lightheadedness (10.3%), excitement (3.7%), and numbness (0.3%). Ninety-one (7.6%) patients became drowsy or fell into a light sleep but all were readily aroused by verbal command. All retained the ability to cough or swallow on command. No consistent or clinically adverse changes were found in BP or pulse rates. The trial supports the concept that this agent is a promising sedative/analgesic for the relief of mild to moderate pain and anxiety. Because of its safety, it is particularly suited to use in prehospital emergency care.