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It has been demonstrated that opioid peptides are involved in the regulation of mammalian reproduction. In our previous studies we demonstrated direct effects of opioids on preimplantation mouse embryos, and hypothesized the existence in preimplantation embryos of receptors similar to opioid receptors in the central neuronal system of adult animals. In the present study we addressed this issue by employing immunocytochemical staining for mu-opioid receptors using antisera raised against the C-terminal portion of the cloned mu-opioid receptors (MOR1, NHQLENLEAETAPLP, 384-398) predicted from the cloned receptor. Diffuse MOR1 immunoreactivity of moderate intensity has been revealed in one-cell embryos, while in follicular cells MOR1 staining was of high intensity and appeared to be associated with plasma membrane. No MOR1 immunoreactivity has been observed in two-cell to morula stages of development. However, blastocysts displayed intense MOR1-labeling that was particularly prominent in cells within the inner cell mass. MOR1-staining was most likely specific because preincubation of MOR1 antisera with cognate peptide completely abolished the staining. Our findings suggest the presence of opioid receptors during preimplantation development, long before the formation of the nervous system. Embryonic opioid receptors may play a role in the regulation of preimplantation development and implantation.