The relation between diencephalic infundibulum and notochord has been studied in Bufo bufo, either in the normal development or after treatment with thiocyanate. This research has shown that, in the first stages of the normal development (from 18 up to 20) the notochord tip lays upon the diencephalic infundibulum, and between this and the mesencephalon. In the following stages the notochord and the infundibulum separate, and the basal plate develops between them. In the meanwhile, the following facts occur: 1) The roof of the foregut changes its convex shape into flat. 2) The rostral part of the animals develops and this brings the cephalic end far from the telencephalon. 3) Some of the individuals treated with NaCNS at the stages of blastula and neurula display penetration of the notochord into the diencephalic infundibulum , and even into the walls of the neural system, sometimes pushing on near the eyes. The rostral part of the individuals treated at blastula and neurula stages is significantly shorter than the one of the controls, and the roof of the foregut keeps its convex shape. The length of the rostral part of the individuals treated at neurula stage does not significantly differ from that of the controls, and at the same time the morphogenesis of the roof of the foregut does not differ from that of the controls. The treated animals are significantly shorter than the controls. The volume of their notochord does not significantly differ from that of the controls. The whole of these results shows that the extension of the notochord into the nervous system is not due to a hypertrophy of the notochord anlage, but to the united effect of a complex of general inhibitions. In short, such anomaly may be explained as follows: due to an inhibition of some morphological processes the notochord does not separate from the infundibulum and penetrates in various ways into the nervous system because of the decrease in length of the animal not followed by a change of the notochordal volume.