The levels of progesterone in the systemic plasma of eight normal and two ovariectomized cows were estimated by gas chromatography. During the last month of pregnancy, levels varied mostly in the 2·5 to 7·5 ng/ml range (0·5 to 3·0 ng/ml at calving). For six of the eight cows, there was a significant fall in level during this period; pooling of the data for all eight showed a highly significant fall.
Levels near the time of ovulation, whether or not this was associated with behavioural oestrus, were below 2 ng/ml ; they rose and fell during ovulation cycles as corpora lutea grew and regressed. Mean peak level for twenty-two cycles was 9·0 ng/ml and occurred on average 13 days after ovulation. The time of most rapid fall in progesterone level was, on average, 4 days before ovulation.
Mean levels during the first 14 days after oestrus in normal cycles were not different from those during the first 14 days after insemination in early pregnancies; they then declined in the cycles but not during the early pregnancies.
Mean progesterone levels in the plasma of ovariectomized cows up to 200 days after the operation were below 2 ng/ml; levels showed a slow but significant rise during this period.
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