Plasma estrogen, progestin, and luteinizing hormone during the normal menstrual cycle in the baboon: role of luteinizing hormone.

Plasma levels of estrogen, progestin, and LH were determined in 10 female baboons throughout the menstrual cycle and in five baboons during the midcycle. The peak in plasma estrogen occurred either on the day prior to the plasma LH peak in three of the 10 cycles, on the day of LH peak in six cycles, or on the day after LH peak in one cycle studied. During the luteal phase, no plasma peak of estrogen was observed. The plasma level of progestin was low in the follicular phase; it rose significantly on the day of or the day after the plasma peak of estrogen. It developed a small peak and then, after an abrupt drop, reached a plateau 5 to 6 dvels at midcycle was usually observed on the day following the LH peak. Three significant rises in plasma level of LH were observed: at the onset of menstruation, just before, and after ovulation. This observation suggests that LH may play an important part in initiating follicular maturation, ovulation, and luteinization during the normal menstrual cycle in the baboon.