Ovarian follicular development was studied in the rat during a 15-day period preceding first ovulation. Ovaries were obtained by unilateral ovariectomy performed at various ages and the rats were allowed to live until the day after first ovulation. The timing of this ovulation was compared with that in unoperated, paired control rats of the same age. For estimation of gonadotrophin levels, blood was taken from the paired control rats at the time when experimental rats were unilaterally ovariectomized. There was no evidence that unilateral ovariectomy had any influence on the timing of first ovulation. Therefore the ovaries obtained could be dated in relation to first ovulation, and follicular growth during the final prepubertal period could thus be studied in a genuine developmental sequence. Results revealed that follicular growth leading to first ovulation starts at ±8 days before this ovulation; follicular processes taking place are comparable to those found during the adult 5-day cycle but proceed more slowly.
Gonadotrophin concentrations accompanying the follicular dynamics and measured at 11·00 h, showed a clear tendency for FSH concentrations to decrease with increasing age, i.e. approaching first ovulation. Concentrations of LH did not show a definite pattern and were generally low, although in some individual rats relatively high LH values (> 100 μg/l) were found in the period of 5–3 days before first ovulation.