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The progesterone concentration in the peripheral plasma was measured sequentially in individual ewes during the estrous cycle, and during gestation and parturition in intact and in ovariectomized ewes. Progesterone levels during anestrum and in ovariectomized ewes have been measured. During the estrous cycle the concentration was lowest (0.1–0.2 ng/ml) from 36 h before to 48 h after, the onset of estrus. A similar concentration was found during anestrum and in ovariectomized ewes, suggesting that at estrus the progesterone comes from nonovarian tissue, probably the adrenal gland. On the 4th day of the cycle (day of estrus = 1st day) the progesterone level began to rise reaching a peak of 1.9–4.0 ng/ml on the 10th day. After a decline, the level rose to a second peak on the 14th or 15th day. Three to 4 days before the next onset of estrus the concentration dropped sharply over a period of 48 h to a low basal level (0.1–0.2 ng/ml). During early pregnancy the plasma progesterone concentration remained fairly constant at a level similar to the maximum level found during the cycle. A sharp rise started around the 80th day, reaching 15–20 ng/ml around the 110th day. This was followed by a second peak, then a decline in the plasma progesterone concentration before parturition, but the time at which this began was variable (1–11 days) and even on the day of parturition the level was generally &gt; 3 ng/ml. A basal level of &lt; 0.5 ng/ml was reached within 24 h after parturition. Pregnancy in ewes ovariectomized soon after conceiving was maintained with implants of progesterone. When these implants were removed around the 90th day of pregnancy the plasma progesterone concentration showed a similar rise to that which occurred in intact pregnant ewes at this time. Parturition occurred normally in these ewes. Within the limited population sample, any relation between the plasma progesterone and the number, sex, or weight of the fetus (es) was masked by the individual variability in the concentration of progesterone.

SEQUENTIAL CHANGES IN PLASMA PROGESTERONE LEVELS IN THE EWE DURING THE ESTROUS CYCLE, AND DURING PREGNANCY IN INTACT AND OVARIECTOMIZED SHEEP