Levels of progesterone in peripheral plasma samples showed a mean (± S.E.M.) ovarian cycle length of 28 ·63 ± 1·01 days (n= 19).
The preovulatory, or follicular phase (mean ± S.E.M. length: 8·25 ± 0·30 days, n= 56) was defined as that period of the cycle during which progesterone levels remained below 10 ng/ml. The postovulatory, or luteal phase (mean ± S.E.M. length: 19·22 ± 0·63 days, n= 48) was defined as the remaining period of the cycle during which levels remained between 10 ng/ml and 150 ng/ml.
The day of ovulation (day 0) was defined as that preceding the day on which progesterone levels first exceeded 10 ng/ml (day l), at the onset of the luteal phase. Oocytes and preim-plantation embryos recovered from the reproductive tract provided supporting evidence for the timing of ovulation.
The short follicular phase indicated that follicular growth may be initiated during the previous luteal phase. The long luteal phase may be related to the extended period of preim-plantation development.