In Bufo bufo japonicus catecholamine neurons in the preoptic recess organ (PRO) became detectable at the metamorphic climax by formaldehyde-induced fluorescence (Falck-Hillarp technique). In hypophysectomized tadpoles metamorphosis was inhibited and no fluorescent neurons appeared in the PRO. Implantation of a pituitary graft to the hypophysectomized tadpoles induced metamorphosis and development of PRO catecholamine neurons. Administration of corticosterone to hypophysectomized tadpoles resulted in the development of PRO catecholamine neurons in spite of the unmetamorphosed state. On the other hand, prolactin administration had no effect on the PRO neurons of hypophysectomized tadpoles. From these results, in conjunction with our previous results indicating that thyroxine treatment induces development of the PRO catecholamine neurons in thyroidectomized animals but not in hypophysectomized animals, it is concluded that corticosterone is a primary hormone for the development of PRO catecholamine neurons in toad tadpoles.