International Standard Gonadotrophin from pregnant mare serum was assayed by 5 methods: effects on wt. of seminal vesicles, ovaries and uteri, degree of luteinization and estrous smear. A previous study of the International Standard Gonadotrophin from pregnancy urine permits a comparison of the 2 gonado-trophins. One I.U. of PMS is approximately equivalent to one I.U. of PU, the latter being slightly more active. At higher dosage levels the effects of equal doses are not the same, thus, 30 I.U. of PMS produces ovaries 6 times as large as 30 I.U. of PU. On the basis of simplicity, sensitivity and objectivity a combination of the uterine wt.[long dash]estrous smear technics appears to be the most satisfactory assay method.