Abstract
SUMMARY: The content of progesterone was determined in the peripheral venous blood of cows from the 32nd day of pregnancy until the day before calving. The level ranged from 0·74 to 0·98 μg progesterone/100 ml. plasma throughout the 32nd-256th day period, but thereafter a marked decrease was observed, and on the day before calving the level was 0·1–0·4 μg/100 ml. plasma.

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