The action of the hormones of the principal and neurointer-mediary lobes on the hypophysis of the toad Bufo arenarum on mammals was studied. The hypophyses were suspended in a saline soln. and injected to hypophysectomized dogs with surgically reduced pancreas (1/5 of its original wt.), guinea pigs, pigeons and normal or hypophysectomized rats. The anterior lobe of the hypophysis of this toad had definite hormonal actions when injected into mammals. In the order of decreasing activity these were diabetogenic (very intensive), growth (very intensive), thyrotrophic, adrenotrophic, increase in wt. of thymua and spleen, and prolactin (very poor). A gonadotrophic action in mammals was not demonstrated for the toad pituitary (even with 66 lobes a day). The neuro-intermediary lobe had a growth, thyrotrophic, adrenotrophic factor, and wt.-increasing action on the spleen. These actions were much less pronounced than the corresponding of the ant. lobe. Thymotrophic and gonadotrophic actions could not be demonstrated.