Human mammary ducts were maintained in median-term organ culture. DNA synthesis was assessed by the uptake of 3H-thymidine in epithelial cells; labeling indices were counted on autoradiographies. DNA synthesis grossly remained between starting values in the basic conditions without hormones. Insulin (5 µg/ml) did not stimulate the uptake of the isotope as compared with the controls. Estradiol (0.001 µg/ml) and progesterone (1 µg/ml) highly significantly enhanced DNA synthesis; subsequent waves did occur.