This investigation is a series of four experiments that examine the role of intrauterine steroidogenesis in the pregnant gilt. In chapter 3, ovariectomy (OVX) and medroxyprogesterone acetate (MPA) treatment at two stages of gestation (d19-29 or d59-69) resulted in maintained pregnancy, normal fetal survival rates, normal conceptus development, and typical plasma estrogens when compared to intact, control gilts. In chapter 4, intrauterine steroid synthesis was quantitated by incubating placenta (PLAC) and endometrium (ENDO) from the control and MPA-treated gilts of chapter 1. Placental P₄, estrone (E₁), and estrone sulfate (E₁SO₄) concentrations were significantly higher than ENDO. Progesterone (P₄) production increased between d30 and 70 of pregnancy while E₁ and E₁SO₄ decreased. The addition of pregnenolone (P₅) to the incubation medium enhanced P₄ but not E₁ or E₁SO₄ release. MPA-treatment had no effect on in vitro steroid production. In chapter 5, OVX gilts from 9 stages of gestation (d20, 25, 30, 35, 40, 45, 55, 60, 80 and 90) were administered P₅ until undergoing hysterectomy (10d after OVX). Only 1 of 10 gilts OVX on d20 or 25 was able to maintain pregnancy for the entire treatment period. The pregnancy rate was variable (67-100%) for gilts OVX between d30-45 of gestation and 100% for gilts OVX subsequent to d45. All measures of whole litter survival and conceptus development for gilts that maintained pregnancy were equivalent to those expected under ovarian-intact, untreated conditions. Plasma steroid levels were relatively normal but allantoic P₄ failed to increase late in gestation. In chapter 6, in vitro steroid synthesis by PLAC and ENDO from gilts treated in chapter 5 was evaluated. Placental P₄ production increased as gestation progressed while ENDO P₄ production was low throughout. The addition of P₅ to the incubation medium resulted in increased P₄ synthesis for both tissues at most stages of gestation. Extending the incubation period also resulted in increased P₄ production at several stages of pregnancy. In vitro estrogen production increased markedly as gestation progressed past d65. The addition of P₅ and extended incubation time enhanced E₁ but not E₁SO₄ synthesis. Overall, data indicate that the PLAC and ENDO have a large capacity for steroid synthesis and estrogen synthesis can occur de novo in the absence of ovarian precursors. / Ph. D.