A single injection of PMS induces ovulation in immature rats on the third day morning after the treatment. The second estrus occurred 9 days after the first ovulation. Follicles of the succeeding cycle developed the capacity to ovulate 3 days after the first ovulation in 3 out of 5 rats, but could not be maintained in an ovulable state during the next 24 hr.
When supplementary PMS was given just after the first LH surge, follicles developed the capacity to ovulate within 1.5 days after the first ovulation and the second ovulation occurred in 7 out of 10 on the fourth day morning after the first ovulation. Vaginal smears showed a 4-day-cyclic pattern in these animals after supplementary PMS administration.
From these experiments, it is concluded that immature rats pretreated with PMS were in an anestrous state during 8 days after the first ovulation, not due to the activation of corpora lutea, but due to the failure of follicular development. Plasma levels of gonadotropin responsible for follicular growth and maintenance after the first ovulation were discussed.