The carcinogenicity of cyanoguanidine, a compound used in the production of melamine, guanidine salts and guanamine derivatives, was examined in male and female Fischer 344 rats fed CRF-1 pulverized diets containing 0, 2.5 and 5% cyanoguanidine for up to 2 yr. The rats were randomly allocated to three groups, each consisting of 50 males and 50 females. The mean body weight gains in both sexes of the 5% group and in females of the 2.5% group were significantly lower than the control values after wk 1 of treatment. No other signs of toxicity were seen in any of the rats throughout the treatment period. Histopathologically, various tumours developed in all groups, including the control group, but these were all similar to those known to occur spontaneously in this strain of rats, and no toxicologically significant increase was found for any lesion type in the treated groups. On the basis of these results, it is concluded that cyanoguanidine exerts no carcinogenic potential in F344 rats when administered for up to 2 yr under the conditions of the present study.