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Atresia of large preovulatory follicles was studied in rats with 5-day reproductive cycles in which ovulation was blocked by two injections of sodium pentobarbital (Nembutal) at proestrus and estrus, respectively. The first morphological signs of atresia appeared 3 days after the first injection of Nembutal, After 4 days the oocytes of all atretic follicles showed resumption of meiosis.
Uilenbroek et al. (Fri,) studied this question.