The influence of chlormadinone acetate treatment on the concentration of some steroids in the blood, on the ovarian activity, and on the sexual behaviour of the mare.

The effect of orally administered chlormadinone acetate (CAP), 10 mg daily for a period of 16 days, was investigated in the case of four mares with an irregular oestrous pattern accompanied by low ovarian activity (group 1), four mares which did not show oestrous symptoms at all and which had also low ovarian activity (group 2), and two ovariectomized and two ovario-hysterectomized mares (group 3). In all mares of group 1 and in the two ovariectomized mares of group 3 oestrus symptoms became apparent during treatment. Two mares of group 2 came into heat 8 and 11 days after the cessation of treatment. The two other mares of group 2 and the ovario-hysterectomized mares of group 3 showed no oestrous symptoms at all. In all except the two ovario-hysterectomized mares the progesterone concentration was found to increase in the peripheral blood from less than 1 ng/ml up to at least 4-6 ng/ml. In the mares which came into oestrus this progesterone increase occurred about three days before symptoms of heat became apparent. It is assumed that the uterus played an important role in the increase of the progesterone concentration in the blood during or after CAP treatment. A similar rise of the progesterone level was also found in the ovariectomized mares, indicating that this progesterone did not originate in the ovaries.