Estrogen production by the preimplantation equine embryo is presumed to be important in maternal-conceptus communication in the mare. The synthesis of C₁₈ estrogens from C₁₉ androgens requires cytochrome P450 aromatase (P450_arom) in the conceptus, but little information is available on the specific tissue location or potential developmental patterns of expression for the horse. The goal of this research was to localize P450_arom in the equine conceptus by immunocytochemistry and in situ hybridization. Intact blastocyst-stage embryos were collected by nonsurgical flush on Days 12–15 of pregnancy, fixed in 4% paraformaldehyde, and paraffin-embedded. Aromatase protein was localized using rabbit anti-human placental aromatase antiserum with a detection system utilizing peroxidase and 3-amino-9-ethylcarbazole. For in situ hybridization, tissue sections were incubated with sense or antisense [³⁵S]UTP-labeled cRNA probes prepared from equine aromatase cDNA. Aromatase protein and transcript were abundant in the extraembryonic trophectoderm but absent from embryonic ectoderm. No P450_arom expression was detected in abembryonic endoderm or mesoderm. Aromatase expression was demonstrated in the endoderm beneath the disc (hypoblast). This pattern of P450_arom expression in the equine blastocyst closely resembles that seen transiently in the porcine embryo, suggesting that regulatory mechanisms conferring tissue specificity may be conserved.