To examine whether there is a possible inhibitory effect of melanophore-stimulating hormone (MSH) on the release of MSH from the pars intermedia, two groups of frogs, Rana nigromaculata, were prepared with median eminence extirpation to eliminate the inhibitory hypothalamic control over MSH secretion. In one of the two groups, the animal's own intermediate lobe was transplanted to the anterior chamber of the eye (intraocular autotransplanted gland), and in the second, the animal's own pars intermedia was left in situ, and another pars intermedia was homotransplanted intraocularly. The MSH content of the homotransplanted pars intermedia was usually much increased and the number of secretory granules augmented in the secretory cells in comparison with the autotransplanted gland. These results lead to the conclusion that the release of MSH from the intraocular pars intermedia is inhibited by the in situ pars intermedia of host animals and indicate that MSH acts directly at the pituitary level to inhibit its further release, thus supporting the concept of mass action-type feedback control.