The gonadotropic potency of avian hypophyses was determined by injecting a suspension of acetone-dried glands into immature female rats. The degree of response was measured by the change in the histology of the epithelium of the vagina;On the basis of available evidence, the approximate threshold amount of acetone-dried pituitary material (whole gland) required to produce stage 3 (the vaginal unit) in the vaginal epithelium of this particular strain of rats was as follows: (a) Capons---7.5 mg. (b) Springs (2.5 to 3.5 lbs.)---14.2 mg. (c) Roasters (3.5 to 6.0 lbs.)---14.6 mg. (d) Turkeys---19.4 mg. (e) Pullets (approaching sexual maturity)---26.0 to 27.5 mg. (f) Fowl (medium to poor fecundity)---34.3 mg. (g) Fowl (good production)---40.0 mg. One positive response was obtained by injecting 25.0 mg. of Pekin duck hypophyses;Sufficient pituitary powder was not available to determine the rat vaginal unit for old cocks, geese, guineas and pheasants, but the indications were that the hypophyses of these birds did not contain appreciable gonad-stimulating properties;Age and state of reproductive activity have a pronounced effect on the gonadotropic potency of avian pituitaries. In general the glands of young birds (springs) are more potent than older birds and the glands of non-laying females are more potent than the hypophyses of laying hens;The follicle-stimulating and luteinzing potency of the hypophyses obtained from male birds was greater than obtained from females providing age was considered.