Follistatin-related gene (FLRG) was first identified as a target of a chromosomal translocation in a human B-cell leukemia. Because FLRG protein binds to activins and bone morphogenetic proteins, FLRG is postulated to be a regulator of these growth factors. However, physiological aspects of FLRG are unclear. To elucidate the physiology of FLRG, we examined expression of FLRG in reproductive tissues of the rat. FLRG mRNA was abundantly expressed in the placenta. FLRG mRNA was also expressed in the ovary, uterus, testis, lung, adrenal gland, pituitary, kidney, small intestine, and heart. During the second half of pregnancy, expression of FLRG in the placenta continuously increased, whereas follistatin mRNA levels decreased from Day 12 to Day 14 and remained low thereafter. FLRG was also expressed in decidua. Levels of decidual FLRG mRNA remained low from Day 12 to Day 16 and then noticeably increased until Day 20. In contrast, follistatin mRNA was highly expressed in the decidua on Day 12, continuously decreased until Day 16, and then remained at relatively low levels thereafter. During the rat estrous cycle, levels of ovarian FLRG mRNA fluctuated diurnally, with highest levels during daytime, and did not change relative to the day of the estrous cycle. The present results suggest that FLRG may play a role in the regulation of reproductive events.