The author discusses the role of "natural" hepatotoxic substances, derived from plants and fungi, in the etiology of liver disease, especially in tropical and subtropical countries. The hazards involved in even the occasional use of natural hepatotoxins and the difficulty in tracing the causative factors of chronic diseases are illustrated by the example of the pyrrolizidine (Senecio) alkaloids. The ingestion by rats of a single dose of these alkaloids can induce chronic liver lesions and even hepatoma which, however, may not become apparent for 1(1/2)-2(1/2) years.It is suggested that, however varied the chemical structures of the various hepatocarcinogens, they may all affect an essential cell constituent (e.g., a "mitotic hormone"), possibly of a steroidal nature, each substance interfering with a particular stage of its biosynthesis. "Natural" toxic factors may also be responsible for some other chronic diseases, especially those which are mainly encountered in, or restricted to, certain pastoral communities. In view of the greater susceptibility of the suckling young and the foetus than of adults to hepatotoxins, it would appear more promising to attempt to trace the causative agents of liver disease in children than in adults, in whom disease takes much longer to develop.