2-Phenylamino-6,8-dibromo-4H-3,l -benzoxazinone has been reacted with nitrogen nucleophües, such as hydrazine hydrate, amines and formamide. and yielded 4(3H)quinazolin-one derivatives; and with sulphur nucleophiles and yielded the corresponding thioesters. The behaviour of aminoquinazoli-none and 4(3H)quinazolinone tovvards carbon eleclrophiles under different conditions has been described.