Isocaloric diets, containing 8% casein [low protein (LP)], 19.5% casein [normal protein (NP)], or 31% casein [high protein (HP)] and 10% corn oil, were fed to adult virgin female Sprague-Dawley rats prior to conception and were continued through mating, gestation, and lactation. Female pups, 8 per litter, were fed on their mother's diet for the duration of the study. The pups' sexual maturation occurred at 33, 35, and 70 days of age for the HP, NP, and LP groups, respectively. At 7 weeks of age, duct development in the mammary gland was markedly delayed in the LP group. No significant differences in the extent of ductal development were noted between the HP and NP groups. Morphologic development into terminal end buds, alveolar buds, lobules, and terminal ducts was determined for each diet group at 7 weeks and at sexual maturity plus 3 weeks (SM+3) of age. At 7 weeks, the terminal end bud was the dominant structure in the LP group and was significantly greater in number than in the HP and NP groups. The latter groups were dominated by the alveolar bud structures. At SM+3, the number of terminal end buds decreased, and the alveolar buds increased in the LP group; however, the numbers were still significantly different than in the HP and NP groups. This study indicates that dietary protein affects mammary duct proliferation and morphologic development.