Recently we have shown that nitrous oxide (N2O) was able to block the expression of morphine-induced conditioned place preference (CPP) in mice. Because dopamine (DA) has also been associated with the positive place conditioning we hypothesize that exposure to N2O would be significantly associated with a modification of extracellular level of DA. Unbiased place conditioning method was used for mice and rats. Levels of DA, in the nucleus accumbens (Nac), in awake and freely moving rats during positive place conditioning after morphine chronic treatment has been measured by microdialysis. Expression of morphine-induced CPP was totally abolished in mice and rats exposed to N2O. Results of animals placed in the morphine-paired compartment showed a 75% increase in the extracellular levels of DA, which was blocked by exposure of animals to N2O. In conclusion we showed the capacity of N2O to block the expression of morphine-induced CPP in mice and in rats. Then we demonstrated an increase of DA extracellular level in the Nac when animals were placed in the morphine-paired compartment and these increase of DA level was blocked by N2O.