It has been shown in previous publications that sex hormones are present in the blood serum of mares during welldefined periods of pregnancy ( Cole and Hart, '30, '30 a). An ovary-stimulating hormone (or hormones) is present as early as the thirty-seventh day and uniformly present by the forty-second day following fertilization. This hormone is present in the circulating blood until about the 150th day of pregnancy. We have no experimental data which would give evidence regarding the source of this ovary-stimulating principle. We have already drawn attention to the similarity of the ovarian reaction in the rats injected with the serum to that obtained by Smith and Engle ('27) with hypophyseal implants. However, the reactions are not identical. We have found that ovulation following injection of blood serum of mares is comparatively rare.
After about the 150th day the serum has an inhibiting effect upon the ovaries. It still has a stimulating effect upon the uterus and vagina, due no doubt to the presence of oestrin.'
' The presence of these hormones makes it possible to diagnose pregnancy accurately from the fortieth day until approximately the end of the gestation period. Zondek ( '30) advocated a method of diagnosing pregnancy in the mare by determining the concentration of oestrin and the follicle-stimulating hormone in the urine. We have found his method more tedious than that proposed by US with the use of serum. Recently the urine of a mare 264 days pregnant was tested with and without the treatment proposed by Zondek. We were surprised to find the urine highly reactive even though untreated. I n the untreated urine 8000