Forty-six Hereford heifers were slaughtered at different stages of gestation. Udders and gravid uteri were removed. Each gravid uterus was dissected into fetus, fetal membranes, fetal fluids and uterus. Fresh weights, dry matter, ether extract, ash, nitrogen and gross energy contents of the various tissues were determined. Weights and compositions of udders and uteri from 36 non-pregnant heifers were also determined. Relationships of the type W = Woe (b~-b2t) t where t is the number of days after mating, Wo is the amount of constituent on the day of mating and W is the amount of constituent on day t of gestation were derived to describe growth of different constituents of the gravid uterus during gestation.Udder development was most rapid after 234 days of gestation as indicated by ash and nitrogen contents. Udders of pregnant heifers contained approximately 48, 60, 67 and 102 g nitrogen at 134, 189, 237 and 264 days of gestation, respectively and udders of non-pregnant heifers contained approximately 37 g nitrogen. No net accumulation of energy occurred in udders of pregnant heifers re/ative to that gained in udders of non-pregnant heifers.Fetuses averaged 1.58, 7.53, 20.8 and 31.6 kg and contained 1.20, 1.60, 2.07 and 2.52% nitrogen at 134, 189, 237 and 264 days of gestation, respectively. Gross energy contents of fetuses were .54, .88, 1.17 and 1.32 kcal/g fresh weight at each of these times. Rates of nitrogen and energy storage in the gravid uterus were estimated to be .76, 2.79, 8.95 and 25.0 g/day and 36.0, 143,457 and 1,167 kcal/day at 100, 160, 220 and 280 days of gestation, respectively. None of these values was affected by fetal sex or maternal energy intake (150 or 1Present address:
