The mammalian uterus can accept a developing blastocyst for implantation only within a limited period of time, termed the receptive phase. Our previous studies showed that the expression of calcitonin, a peptide hormone that regulates calcium homeostasis, is induced by progesterone immediately preceding implantation, and is required for the generation of a receptive rat uterus. In this study, we investigated the expression and hormonal regulation of calcitonin in the baboon endometrium during the window of implantation. We monitored the spatio-temporal expression of calcitonin at various days of the menstrual cycle. Reverse transcriptase-polymerase chain reaction analysis of the baboon endometrium on Days 9 and 10 postovulation revealed stage-specific expression of calcitonin mRNA, which overlapped with the window of uterine receptivity. Immunocytochemical analysis of baboon endometrium sections localized calcitonin expression in the glandular epithelial and stromal cells. Treatment of animals with the antiprogestin ZK 137.316 dramatically reduced calcitonin expression, indicating that calcitonin expression in the baboon endometrium is under progesterone regulation. Collectively, these findings strongly suggest that the appearance of calcitonin in progesterone-dominated endometrium is conserved among species and may serve as a marker of uterine receptivity for embryo implantation.