Effects of age and ovariectomy (OVX) at birth on uterine growth, endometrial development, and estrogen receptor (ER) expression were determined for intact and OVX gilts (n = 5 per day) hysterectomized on postnatal days (PND) 0, 15, 30, 60, 90, or 120. Uteri were evaluated histologically, and ER protein and mRNA expression were characterized immunohistochemically and by in situ hybridization. OVX did not affect uterine weight or endometrial thickness until after PND 60, when both increased more rapidly in intact gilts. Neither did it affect genesis of uterine glands, which were present and which proliferated after PND 0, or endometrial ER expression patterns in glandular epithelium (GE), luminal epithelium (LE), or stroma (S) between PND 0 and 120. Endometrium was ER negative at birth. On PND 15, the ER signal was strong in GE, weak in S, and effectively absent in LE. Thereafter, although the ER signal remained strong in GE and increased through PND 60 in S, it was not evident consistently until after PND 30 in LE. The data indicate that 1) porcine uterine growth and endometrial morphogenesis are ovary-independent processes before PND 60; 2) uterine gland genesis is associated temporally with development of ER-positive endometrial GE and S; and 3) regulation of endometrial ER expression is ovary independent between PND 0 and 120. The results establish the ER as a marker of GE differentiation and implicate this receptor in mechanisms regulating endometrial morphogenesis in the neonatal pig.