Electrical tooth stimulation was used to investigate whether humans develop tolerance to nitrous oxide (N(2)O) analgesia within a single administration as well as over repeated administrations. In a double-blind cross-over experiment, 77 subjects received a 40-min administration of 38% N(2)O at one session and placebo gas at the other. The sessions were separated by 1 week and the order of gas administration was counterbalanced. Acute analgesic tolerance developed for pain threshold but not for detection threshold. There was no evidence of a hyperalgesic rebound effect following cessation of the N(2)O administration. In a second double-blind experiment, 64 subjects received both 30-min of placebo gas and 30-min of 35% N(2)O, separated by a 35-min gas wash-out period, during each of five sessions. Sensory thresholds were assessed prior to drug or placebo administration (baseline) and between 7-12 and 25-30 min of gas administration. A control group of 16 subjects received only placebo gas at these five sessions. During a sixth session, the experimental procedures were similar to the previous sessions except that the control group received N(2)O for the first time and the experimental group was sub-divided to test for conditioned drug effects. For both detection and pain threshold measures, acute tolerance developed during the initial N(2)O exposure and chronic tolerance developed over repeated administrations. Although chronic tolerance developed, a test for Pavlovian drug conditioning found no evidence of conditioned effects on sensory thresholds. In conclusion, acute and chronic tolerance develop to N(2)O's analgesic effects in humans.