The pyrimidine ring of 7-amino [1, 2, 5] thiadiazolo [3, 4-d] pyrimidine (Ia) has been opened by aqueous base and by primary amines, the latter agents producing N-alkyl carboxamidines. During studies on possible modes of ring cleavage, 7-pyrrolidino [1, 2, 5 Ithiadiazolo [3, 4-d] pyrimidine (IC) was treated with dilute hydrochloric acid in dioxane at 85". The product had the composition of the hydrochloride of the amino carboxamidine IIc, ...