5-Aminofurazano [3, 4-d] pyrimidines carrying a variety of substituents at position 7 suffer ring cleavage by either acid or base to give 4-guanidino-3-furazancarboxylic acid (6), the esters of which can be recyclised to give the pterin analog 5-amino-7 (6H)-furazano [3, 4-d] pyrimidinone (9). The pyrimidine ring of (9) is cleaved by hydrolysis, and the furazan ring by hydrogenolysis.