The time course of nitrous oxide analgesia was studied in rats with a behavioral criterion, the tail-flick test to radiant heat. All rats were placed individually in a Plexiglas® tube and exposed to either nitrous oxide, 75% in oxygen, or room air (control) for 2 hr. Analgesic potency was evaluated by prolongation of the time required to induce tail-flick. Although individual animals showed variability in the tail-flick time during exposure to nitrous oxide, no animal showed a tendency toward the development of tolerance, and a statistically significant sustained prolongation of tail-flick time was produced.