The reaction of 3-phenyl-5-aminoisoxazole-4-carboxamide with acyl chlorides gave variable amounts of 3-phenyl-4-cyano-5-acylaminoisoxazoles and 3-phenyl-6-alkyl(or aryl)isoxazolo-[5,4-d]pyrimidin-4(5H)-ones. Some reactions of the latter and transformations in the pyrazolo[3,4-d]-pyrimidine ring system have been described.