Changes in progesterone levels in milk and in milk fat were measured by radioimmunoassay in nine crossbred cows for 40 days after insemination. On average, progesterone in milk was minimal: 1·10μg/l at the morning milking (M) and 1·63μg/l at the evening milking (E) on the day of oestrus; this increased to 15·72 μg/l at M and 22·8 μg/1 at E on day 13 after insemination, and thereafter declined to 0·99 μg/l at M and 1·12μg/l at E on the day that the next oestrous cycle commenced in non-pregnant cows. In pregnant animals the level was maintained, and fluctuated from 1505 to 19·60μg/l at M and 18·09 to 23·01 μ/1 at E up to 40 days of pregnancy.
The average progesterone concentration in milk fat was 18·9μg/l at M and 24·1 μl at E on the day of oestrus, and rose to a peak of 141·7μ/l at M and 153·8μg/1 at E on day 13 after insemination. It continued to increase and fluctuated between 160·6 and 195·8μg/l at M, and 161·1 and 225·6μg/l at E, in pregnant animals, but dropped to 12·0μg/1 at M and 16·7 μg/l at E on the day of the succeeding oestrus in those animals which failed to conceive. Progesterone concentration in milk and in milk fat was significantly higher in evening samples than in morning samples (P<0·05).