DURING the normal development of Amphibians the well-known ‘;fields’ of organization operate to induce morphological differentiation1. Furthermore, adult Urodeles and larval Anurans are known to retain local fields which are active in the regeneration of parts2. Therefore, problems in abnormal growth may well be studied in those Amphibians which retain power of regeneration. One of the commonest present-day methods of experimentally producing abnormal growth is by means of carcinogenic substances, but apparently these substances have been little used for such studies in Amphibia. Recently, however, Koch, Schreiber and Schreiber3 have induced tumours in adults of Triton cristatis and Triton taeniatus with 1: 2 benzpyrene and with carcinogenic tar. The present report deals with induction experiments with Rana pipiens tadpoles using methylcholanthrene-choleic acid as the carcinogenic agent.