After 3 and 5 days of culture of whole rat mammary glands in the presence of insulin, or insulin plus prolactin, or insulin plus prolactin plus aldosterone, or insulin plus prolactin plus oestradiol plus progesterone, there was good correlation between the levels of alveolar development and the [3H]thymidine labelling indices. Blockage of mitosis in explants developing in medium containing insulin plus prolactin also blocked alveolar development. Alveolar development is therefore proportional to and dependent on mitotic activity in the proper hormonal environment.