Ductal morphogenesis of the mammary gland occurs largely in the early pubertal period under the influence of ovarian sex hormones. In this study we characterized developing ductal structures in breast tissue of 14 female prepubertal or early pubertal rhesus monkeys (average age, 2.6 +/- 0.1 years). Serum estradiol concentrations ranged from < 5.0 to 61.2 pg/ml, while progesterone concentrations were low in all animals (< 0.6 ng/ml). No corpora lutea were seen on ovarian histology. The most immature mammary glands contained multilayered ducts frequently ending in distinctive terminal end bud structures. Transitional ducts had variably stratified columnar epithelium, often with prominent rounded myoepithelial cells and loose periductal stroma. Mature ducts had simple cuboidal luminal epithelium, flattened myoepithelium, and associated lobular development, typical of the premenopausal breast. Expression of estrogen receptor alpha was significantly higher in immature ducts compared to mature ducts, while progesterone receptor expression, epithelial proliferation, and cytokeratin expression did not show a relationship to the maturity of the ducts. These findings identify dynamic morphologic changes occurring in the primate breast during early puberty and suggest that estradiol (rather than progesterone) is the predominant hormonal stimulus for early ductal and lobular development.