Adult capons were treated with prolactin (LTH), follicle stimulating hormone (FSH), luteinizing hormone (LH), singly or in combination with a constant progesterone (Pg) dosage. In addition, Pg was administered alone. Pg was effective in activating the castrate feather papilla, these findings confirming earlier responses obtained in both intact sexes of fowl by others. FSH and LH were both ineffective in stimulating capon feather regeneration; in combination with Pg, the reaction was similar to that toward Pg alone. LTH uniformly caused molt of capon feathering whether given alone, in combination with Pg, or alone as a highly purified preparation. The positive performance of LTH in the male castrate feathering described here, together with the observed lack of response toward FSH and LH in this fowl, suggests that prolactin may be the immediately effective agent in stimulating the feather papilla to production of new plumage.