TREATMENT of 3,4-dihydroquinazolin-4-one (2a) with P2S5 ……..yielded the corresponding thione 3 that treated with hydrazine/or 2-aminoethanol afforded quinazoline derivatives 4.The hydrazinoquinazoline 4b converted to (sulphahydryl and/or methyl) triazoloquinazoline via interaction with CS2 and Ac2O. On the other hand, compound 2a reacts with ethyl chloroacetate yielded the ester derivative 9 which converted to the corresponding hydrazide 10 via interaction with hydrazine hydrate. Its behavior of hydrazide 10 towards carbon electrophiles, e.g., thiophene-2-carboxaldehyde, furfural, piperonal, o-anisaldehyde, phthalic anhydride, ammonium thiocyanate, acetyl acetone, Ac2O and ethyl acetoacetate afforded compounds 11-16. Also, behavior of compound 2b towards carbon electrophiles, e.g. acetic anhydride and benzoyl chloride afforded 17. A moderate activity was observed with new quinazolinone compounds 4-10 which proved to possess marked activity against E. coli, S. aureus and C. albicans. The strong activity was observed with compounds 3,11-17.