In view of the tumor inhibitory activity of various pyrimidine derivatives, investigations aimed at the structure modification of barbituric acids were performed. In accordance with the aminomethinylation principle, barbituric acids (2) are converted into 5-formimidoylbarbituric acids (4) by s-triazine (1). Aside from the free barbituric acid (2a), both 1-substituted (2b, d, and f) and 1,3-di-substituted (2c, e, and g) barbituric acids are amenable to this reaction. After introduction of the formimidoyl group into 5-position of the barbituric acid ring system, marked tumor inhibitory effects against lymphoma, lymphocytical leukemia, and luekemia L-1210, are ascertainable.