The administration of sulphonamido derivatives of pyrimidine (e.g. sulphadimidine) to young cockerels induces testicular hypertrophy. Of thirty-five compounds tested for the effect, only phenylsulphonamide derivatives of pyrimidine and quinoxaline have been found to be active. The effect of testicular hypertrophy appears to be independent of bacteriostatic action as it is produced by 2-phenysulphonylamido-4:6-dimethylpyrimidine, and the effect of sulphadimidine is not neutralized by administration of p-aminobenzoic acid or of folic acid. Sulphadimidine reversed the inhibitory effect of stilboestrol on the testes of young rats. Testicular hypertrophy could not be induced regularly in normal young rats or mice by sulphadimidine. The synthesis of some new pyrimidine derivatives is described.