Fructose-1,6-diphosphate (FDP) hydrolysis, which was measured in tissue extracts from mature rat endometrium, was maximal during late dioestrus and early pro-oestrus and reflected the variations in the plasma levels of ovarian steroids. Treatment of ovariectomized animals with oestradiol or medroxyprogesterone acetate increased FDP hydrolysis when compared with control animals, although oestradiol was the more effective. FDP hydrolysis was greater in extracts of endometrial tissue from uterine horns containing a silk IUD or of deciduomal tissue from pseudo-pregnant rats, in which the peak was on Day 9, than in extracts of endometrium from the control contralateral horn. These findings are interpreted as linking glycogen metabolism in normal endometrium, IUD-containing horns, and decidual tissue to the role of FDP in gluconeogenesis and the promotion of glycogen storage.