Aminoguanidine sulfate (AG) inhibits in vivo oxidative deaminations of the polyamines and their derivatives. This compound was used to study urinary polyamine excretion by normal, and tumor bearing rodents. Of the total expendable polyamines, 64 percent were catabolized by AG-sensitive oxidases and escaped observation. Tumor bearing animals did not excrete enhanced amounts of polyamines at any stage of tumoral growth. However, treatment with adriamycin caused an increased polyamine excretion. Prolonged administration of a 2% solution of a-difluoromethylornithine (DFMO), reduced urinary polyamine excretion to the same level of about 27%, irrespective whether the animals carried a large tumor or not. Cadaverine excretion was not affected by treatment with DFMO. Based on these animal data, it appears that urinary polyamines are of restricted value in the diagnosis of tumors.