The aim of the present study was to characterize the binding selectivity of the mu-opioid receptor ligands, endomorphin-1, endomorphin-2, and DAMGO, in the in vitro functional assay, based on the changes in intracellular calcium levels. For the experiments Chinese hamster ovary cells, stably expressing human mu-receptor, were used. The mu-agonist-induced calcium responses were significantly inhibited by naloxone, an opioid antagonist with high preference for the mu-opioid receptors. Naloxonazine, a mu1-non-peptide antagonist, inhibited the effect of all tested mu-agonists. However, there was no significant difference in the antagonist effect of naloxonazine on the calcium response induced by mu1- (endomorphin-2) and mu2-agonists (endomorphin-1, DAMGO). [D-Pro2]endomorphin-1 and [D-Pro2]endomorphin-2, putative peptide mu2- and mu1-antagonists, respectively, which had been shown in vivo to inhibit the antinociception induced by mu-agonists, produced no inhibitory effect in our in vitro experiments. Our results demonstrated that there is only one population of the mu-opioid receptors expressed in the Chinese hamster ovary cells. We suggest that the mu-opioid receptors form a homogenous population in the in vitro systems. However, the existence of mu-receptor subtypes in vivo is still pharmacologically possible