Guppies (Lebistes reticulatus) were used as models for the screening of potentially carcinogenic substances. These fish, a simple, readily accessible type, are sensitive toward carcinogens. Using this model, we performed studies on the morphological effects of a few water-soluble chemicals. Guppies treated with diethylnitrosamine were used as positive controls. Changes were noted in the livers, and a few specific lesions occurred in their gastrointestinal tracts. Studies were also conducted with 10 other similar chemicals found in the human environment that have potential use in chemotherapy. Of these, the carcinogenic effect of N,N'-dinitrosopiperazine on fish was demonstrated. Some researchers think this compound is a derivative of the pesticide Triforine. Furthermore, 2 of the 10 chemicals intended for use in therapy proved to be potential carcinogens. Because these experiments with water-soluble substances are inexpensive and the test periods are short, the use of fish is considered more advantageous than is the use of rodents.