Ovarian follicular development was analyzed in intact and unilaterally ovariectomized cyclic rats. Holtzman rats, followed for three successive cycles, were classified as 4-or 5-day animals, with estrus designated as day 1. Intact 4and 5-day animals were killed on each day of the estrous cycle and one ovary from each animal was saved for histology and examined for normal follicles larger than 352 p, Cycling rats were unilaterally ovariectomized on day 1 and killed on each of the remaining days of the immediate estrous cycle and day 1 of the next cycle. The follicular population (normal follicles larger than 352 p ) was studied in the remaining ovary.
During the estrous cycle of intact rats, there was a gradual increase in the number of follicles with a diameter of 518 to 571 p and greater; however, the totaI number of follicles ranging in size from 352 to 571 p and larger remained relatively constant. At day 5, in the 5-day cycling rat, the ovaries were depleted of follicles measuring from 395 to 570 p.
Following unilateral ovariectomy on day 1, there was a temporary decrease in the number of follicles larger than 448 p. This agreed with the finding that animals semi-spayed on day 1 and injected with 20 I.U. human chorionic gonadotropin on day 2 (PM) did not ovulate, whereas intact animals ovulated an average of seven eggs after the same treatment. The remaining ovary of animals unilaterally ovariectomized on day 1 showed a significant increase by the next estrus in the number of follicles larger than 448 p.
The method of compensatory ovulation in the rat involved doubling the number of large follicles which ultimately matured during the estrous cycle. This resulted from increased proliferation of smaller follicles in the course of the cycle rather than from decreased follicular atresia.