We have examined responses of mice lacking mu, delta and kappa opioid receptor (MOR, DOR and KOR, respectively) genes, as well as combinatorial mutants, in several pain models. This is the first truly comparative study of all three opioid receptor-deficient mice, with genotypes and gender analysis using mice on the hybrid 50% 129/SV : 50% C57BL/6 genetic background. In the tail-immersion test, only KOR-/- females showed decreased withdrawal latencies. This modification was also found in MOR/KOR and MOR/DOR/KOR, but not MOR/DOR mutants. The hotplate test revealed increased nociceptive sensitivity for MOR-/-, a phenotype which was also observed in double mutants involving the MOR deletion, and in the triple mutants. The tail-pressure test showed increased response for both MOR-/- and DOR-/- mutants, a modification which was enhanced in the triple-mutant mice. In the formalin test, MOR-/- and DOR-/- mice showed increased responses in the early and late phases, respectively, while the triple mutant tended to show enhanced nociception in both phases. Finally, the enhanced response of KOR-/- mice in the writhing test, which we have demonstrated previously, was confirmed in double MOR/KOR- and triple-mutant mice. Together, the data support the existence of an antinociceptive opioid tone. Each receptor presents a distinct pattern of activities, with mu receptors influencing responses to mechanical, chemical and thermal nociception at a supraspinal level, kappa receptors involved in spinally mediated thermal nociception and chemical visceral pain, and delta receptors modulating mechanical nociception and inflammatory pain. Phenotypes of mutant mice were subtle, suggesting a low endogenous opioid tone in the regulation of physiological pain.