THE bulk of studies on the response of avian gonads to gonadotrophic hormones has been done using hormones of mammalian origin. This work established that gonads of normal adult chickens of both sexes are capable of being stimulated by mammalian gonadotrophms. However, in the domestic chicken, between the ages of one and about 150 days, the gametogenic component of the ovary is almost completely refractory to gonadotrophins of mammalian origin. Thus, injection of crude or purified gonadotrophic substances from sheep, pigs and mares (including pregnant mare’s serum) does not lead to any measurable enlargement of ovarian follicles of prepuberal female chickens. Two alternative explanations for the failure of the prepuberal avian cortex to respond to mammalian hormones can be offered: 1. a physiological or morphological unreadiness of the cortex of the avian ovary, and 2. an endocrine incompleteness of the mammalian hormones used.