[en] Female albino rats were given, at the age of 8 or 19 days, a signle whole-body exposure to 500 rad on a ''Rocus-M'' apparatus. Mitotic activity of epithelium cells from uterus, cervix, and vagina were examined at 2, 6, 12, 26, 50, and 74 hr postradiation. At 8 days of age (period of active growth), uterine, cervical, and vaginal epithelia had high mitotic indices (28.4%, 21.2%, and 23.0%, respectively). At 2 hr postradiation, mitotic index was found to drop to 3.0%; at 6 hr it began to rise, to attain a maximum at 12 hr (39.8%, 37.0%, 42.9%). A second maximum, of virtually similar extent, was noted at 74 hr postradiation. Over the period from 26 hr to 50 hr, mitotic activity was below maximum. At 19 days of age (period of prefunctional differentiation), mitotic indices of epithelium cells from uterus, cervix, and vagina were 2.9%, 7.1%, and 14.4%, respectively. The radiation induced suppression of mitotic activity in epithelia from the organs studied, as observed at 2 hr postradiation, was succeeded by a gradual rise with a maximum level between 12 hr and 50 hr; this peak rate exceeded control figures by a factor of 5 for uterus and a factor of 2.5 for cervix and vagina. At 74 hr postradiation, cervical and vaginal epithelia had recovered a normal mitotic rate, while uterine epithelium had a mitotic rate still twice the initial figure. The results of the study indicate that recovery of mitotic activity is more rapid and of a higher level for uterine epithelium from animals irradiated at 19 days of age. This is likely to be accounted for by a longer mitotic cycle - 203.4 hr - in uterine epithelium cells, with relatively more extended radiation sensitive phases of the cycle. (author)