We inoculated 214 animals with a 20-Methylcholanthrene suspension in olive oil. The first reaction, 3 to 9 days after inoculation, is an intensive epithelial hyperplasia with cellular disorientation, anisocytosis and increased mitotic activity limited to the site of inoculation. Superimposed on this background of epithelial disorganization an inflammatory process is observed both in treated and control animals. Later a perforation of the basal membrane can occur with invasion of muscle and mesonephros or with pseudotumoral nodular formations. In other cases a papillomatous formation is observed. After 20 days a regressive phase of these processes sets in. The larvae of Bufo arenarum respond to 20-Methylcholanthrene treatment very promptly.