The urinary ketonic estrogens were studied at monthly intervals in five pregnant mares. Estrone and equilin appear to be the principal components and equilenin a minor constituent of the mares’ estrogens. The levels of equilin rise from the 4–5th months to equal and in some instances exceed those of estrone in the late months of pregnancy. A partition chromatographic method for the separation of the ketonic phenolic steroids of pregnant mares’ urine is described.