Abstract Hormone-dependent mammary tumors in GR mice, and the disk-shaped “plaques” from which they originate have been examined by [ 3H]thymidine labeling and electron microscopy. Plaques consisted of long narrow, radiating duct-like tubules lined by dark and light secretory cells containing mammary tumor virus (MTV) and underlaid by a few myoepithelial cells. Autoradiographic analysis of plaques revealed a labeling index of 26%, whereas pregnancy-dependent tumors within the same mice had a labeling index of 38%. At the periphery of some tumors plaque-like tubules were evident. Hyperplasia occurred (1) within some of these tubules and (2) within nearby alveoli, resulting in cords and clusters of tumor cells. Myoepithelial cells could no longer be identified among the tumor cells. Pregnancy-dependent and hormone-induced tumors both contained club-like extensions having distorted lumens. Intracytoplasmic lumina were sometimes observed, as were dark spindle-shaped tumor cells containing MTV. Regression of the tumors was due to rapid cell degeneration which left only a few isolated ducts and/or tubules, some of which presumably have the potential of responding to hormones and initiating another tumor at the same site.