The immunohistochemical and ultrastructural localization of prostaglandin H synthase (PGH synthase) was studied in the Albino Swiss CF-1 mouse at different stages of embryonic development (two-cell stage, four-eight cell stage, morula and blastocyst). Flushed embryos and sections of uteri and oviducts containing embryos were treated with a mouse IgG monoclonal anti-PGH synthase antibody. The second antibody (rabbit anti-mouse) was conjugated with peroxidase or fluorescein isothiocyanate for light microscopy, fluorescence microscopy and confocal scanning. For reflection contrast microscopy and transmission electron microscopy a second antibody, goat anti-mouse, was conjugated with ultrasmall gold particles. Controls without anti-PGH synthase were used concurrently. All embryos demonstrated PGH synthase reactivity. Immunostaining appeared to be more intense at the two-cell stage, four-eight cell stage embryos and morulae than in blastocysts. Further examination indicated an intracytoplasmic location for PGH synthase, which was confirmed by stereoscopic photographs made during confocal scanning microscopy and by the immunostaining patterns observed with reflection contrast microscopy and transmission electron microscopy. Transmission electron microscopy immunostaining patterns support the localization of PGH synthase in the endoplasmic reticulum. This is the first demonstration of the ultrastructural localization of PGH synthase in the mouse embryo. Its presence before the apposition with the endometrial epithelium supports the hypothesis that arachidonic acid metabolism via the PGH synthase pathway may be crucial for implantation.