The recent identification and cloning of guanine nucleotide regulatory protein-coupled brain-type and spleen-type cannabinoid receptors (CB1-R and CB2-R, respectively) provide evidence that many of the effects of cannabinoids are mediated via these receptors. Our recent observation of expression of both CB1-R and CB2-R genes in the preimplantation mouse embryo suggests that it could also be a target for cannabinoids. Indeed, cannabinoid agonists interfered with preimplantation embryo development in vitro. To examine whether cannabinoid effects on preimplantation embryos are mediated via CB1-R, we developed rabbit antipeptide antibodies against the N-terminal region of CB1-R and examined the receptor protein in the blastocyst by Western blotting and its spatiotemporal distribution in preimplantation mouse embryos by immunohistochemistry. Cannabinoid binding sites in the blastocyst were examined by Scatchard analysis, while the reversibility of cannabinoid-induced embryonic arrest in vitro was monitored using a specific antagonist to CB1-R, SR141716A. Western blot analysis detected a major band of approximately 59 kDa and a minor band of approximately 54 kDa in the blastocyst. Immunocytochemistry detected this receptor protein from the 2-cell through the blastocyst stages. Scatchard analysis using 3H-anandamide (an endogenous ligand) showed a single class of binding sites in Day 4 blastocysts with an apparent Kd of 1.0 nM and Bmax of 0.09 fmol/blastocyst. Considering the total number of cells (approximately 50) and total protein content (approximately 20 ng) of a blastocyst, it is apparent that the mouse blastocyst has many more high-affinity receptors than those in the mouse brain (Kd: 1.8 nM and Bmax: 18.8 pmol/mg membrane protein). Cannabinoid agonists and the CB1-R antagonist SR141716A effectively competed for anandamide binding in the blastocyst. To determine whether cannabinoid inhibition of embryonic development could be reversed by SR141716A, 2-cell embryos were cultured in the presence of cannabinoid agonists with or without SR141716A for 72 h. Most of the 2-cell embryos cultured in the absence of the agonists developed into blastocysts (approximately 90%). In contrast, the addition of cannabinoid agonists anandamide, Win 55212-2, or CP 55,940 in the culture medium severely compromised embryonic development: more than 60% of the 2-cell embryos failed to develop to blastocysts. A reduction in trophectoderm cell numbers was noted in those blastocysts that escaped the developmental arrest in the presence of cannabinoid agonists. However, this reduction was corrected when embryos were cultured simultaneously with an agonist and SR141716A. Furthermore, embryonic arrest was reversed when embryos were cultured simultaneously in the presence of an agonist and SR141716A. The addition of SR141716A alone in the culture medium apparently had no effects on embryonic development: more than 90% of the embryos developed into blastocysts. The results suggest that the CB1 receptors in preimplantation mouse embryos are biologically active and cannabinoid effects on them are primarily mediated by these receptors.