Progesterone levels were determined in ovarian venous blood of nine cows with regular oestrous cycles and in three cows with prolonged maintenance of the corpus luteum. The ovarian venous blood was collected by chronic cannulation of the ovarian vein, through the anterior uterine vein. The progesterone level increased from 5·6 μg/100 ml plasma on the 1st day of the oestrous cycle to about 125 μg/100 ml on the 8th day and to about 180 μg/100 ml on Days 14 and 15 of the cycle. Thereafter, the level rapidly decreased to 10 to 20 μg/100 ml on the day of ovulation. In cows with prolonged maintenance of the corpus luteum when ovulation did not occur, the progesterone levels did not decrease and were as high as the highest luteal phase levels in cows with regular cycles. According to the data obtained, progesterone levels in ovarian blood of non-pregnant cows seem to reflect the luteal function of the ovary during the oestrous cycle.