Non-inbred and hybrid mice, line C57Bl and CBA in diestrus stage were subjected to ovariectomy and in 4-6 weeks they were given subcutaneously 17-beta-oestradiol (1 or 0.1 mkg per one animal). One hour before the animals were sacrificed, they were given 3H-thymidine intraperitoneally. It has been stated that 15-20 h after the estrogen administration the amount of DNA-synthesizing cells in the vaginal epithelium of these animals is 25 times as great as that in the control animals--castrated mice. When the epidermal chalone is administered locally or intraperitoneally (5 mg per one mouse), 10 min after the hormone injection, there is no inhibiting effect of the chalone on the proliferative processes in the vaginal epithelium stimulated with estrogen. When a single intraperitoneal injection of the epidermal chalone is given (5 mg per one mouse) 1 h before, or when it is given three times (2 mg per one mouse) 8, 4 or 1 h before the hormone is injected, there is a definite inhibiting effect in the proliferative processes. Their degree depends on how long the chalone was in contact with the cells.