Plasma progesterone was determined with the aid of a competitive protein-binding assay in mares during the oestrous cycle, early pregnancy (45--60 days) and later pregnancy (2--10 months). Progesterone levels were low during oestrus (less than 1 ng per ml) (3,18 nmol/l) and reached high levels (often in excess of 10 ng per ml) (31.8 nmol/l) within 3--4 days after ovulation. The high luteal levels were maintained for approximately 5--8 days and then declined sharply over a period of approximately 24--48 hours to reach low levels at the subsequent oestrus period. In mares conceiving after service, the progesterone levels rose rapidly to 5--9 ng per ml (15,9--28,6 nmol/l) 21 days after service. Levels of 4--10 ng per ml (12,7--31,8 nmol/l) were found between 30 and 60 days after successful service with a tendency towards lower levels from 30--42 days and higher levels from 42--60 days. Progesterone levels remained between 7 and 10 ng per ml (22,3--31,8 nmol/l) from 60--110 days and thereafter fell to a relatively constant level of 3--6 ng pe ml (9,5--19,1 nmol/l) until the tenth month of pregnancy.