Background: The mammary gland is one of the few organ systems in mammals that completes its morphologic development postnatally. Due to this unique developmental property, it serves as an excellent model for studying ductal morphogenesis beyond embryonic life. The primary objective of this study was to investigate the development of terminal end bud (TEB) structure in the prepubertal-pubertal mouse mammary gland. Methods: Seventy-five mice, weaned at 21 days of age, were fed a high fat, nonrestricted intake diet for optimum mammary gland growth. Wholemount preparations and a computer-assisted image analyzer were used to record the size and number of TEB structures in the thoracic and inguinal mammary glands. A light microscope was used to determine the number of cap cells present in select TEB structures. Results: In the prepubertal age group, the size and number (No.) of TEBs in the No. 2 and No. 3 thoracic glands were significantly greater than those in the No. 4 inguinal gland. In the pubertal age group, the size and number of TEBs in the No. 2 gland significantly decreased, indicating that for this particular gland the largest size and greatest number of TEBs are present during the prepubertal growth period. In contrast, the size and number of TEBs in the No. 3 and No. 4 glands remained relatively constant throughout the prepubertal-pubertal growth period. Conclusions: The results of this study show: (1) like the rat mammary gland, there is asynchronous growth among the mouse mammary glands, (2) the maximum size and number of TEBs in the prepubertal-pubertal mouse mammary glands are a function of the gland of origin and the age of the mouse, and (3) the number of cap cells per TEB structure is directly correlated with the size of the TEB structure.