The regional distribution of trophic hormonal activities in the avian pars distalis was studied by transplanting the cephalic, middle and caudal regions of the pars distalis of day-old cockerel donors onto the chorioallantoic membrane of 10 day-old partially decapitated embryos. The bioassay was based on the following parameters: (1) Body growth as assessed by body weights and lengths of third toes and (2) growth and histological maturation of the endocrine target organs (thyroids, adrenals and gonads). The experimental design was comprised of: normal embryos, hypophysectomized embryos, hypophysectomized embryos with one cephalic region graft, hypophysectomized embryos with one middle region graft and hypophysectomized embryos with one caudal region graft. All embryos were examined at the end of the 20th day of incubation. Hypophysectomy severely retards growth, vascular shutdown and yolk sac retraction, feather development, skin opacity and the normal maturation of target endocrine organs. The cephalic, middle and caudal regions of the avian pars distalis contained a growth-sustaining factor iii (STH?) capable of normalizing lengths of third toes in hypophysectomized embryos. The yolk sacs of those embryos which received caudal region grafts were heavier than those of all other groups. TSH, ACTH and FSH were exclusively associated with the cephalic and middle regions of the pars distalis whereas LH originated from the caudal region of the gland. STH appeared to be distributed throughout the entire gland. The hypothalamus did not seem to play a significant endocrine role during this period of avian development. Each region was capable of autonomous hormonal activity. The cells of each region were normal after grafting and did not appear to differentiate into other functionally different cell types.