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Reaction of 2-phenyl-3,1-benzoxazin-4H-one 1 with thiocarbohydrazide in refluxing ethanol affords 2-phenyl-3-thiosemicarbazido-4(3H)-quinazolinone 2. However, thioxotetrazinoquinazoIine 3 is obtained in 79% isolated yield when the reaction mixture is fused in an oil bath at 160 degrees C. Compound 3 is also obtained when compound 2 is heated in an oil bath at 170 degrees C. On the other hand, compound 3 reacts with paraformaldehyde and secondary amines to afford the corresponding quinazoline derivatives 4 and 5, The diazonium salt of 3-amino-2-phenyl-4(3H)-quinazolinone 5 reacts with some active methylene compounds (diethyl malonate, ethyl acetoacetate, malononitrile and acetylacetone) to affords the quinazolinone derivatives 8-11 in good yields. Reactions of 8 with hydrazine hydrate and phenylhydrazine afford the corresponding pyrazolonoquinazolinones 12 and 13, respectively. Reaction of 8 with urea furnishes pyrimidinoquinazolinone 14 in 79% isolated yield. However, reaction of 11 with hydrazine hydrate gives pyrazoloquinazolinane 15 in 83% isolated yield.