Acylation of spiro[piperidine-4, 2'-(1', 2', 3', 4'-tetrahydroquinazolin)]-4'-ones (1d, 4, 6, and 7) was attempted. In all cases, the resulting products were not the corresponding acyl derivatives but two types of products (2 or 3 and 5). Heating 1d, the derivative having hydrogen atoms on the N₍₁₎ and N₍₃₎-positions of spiro[piperidine-4, 2'-(1', 2', 3', 4'-tetrahydroquinazolin)]-4'-ones with acetic anhydride or benzoic anhydride gave 1-(1-benzyl-1, 2, 5, 6-tetrahydro-4-pyridyl)-2-methyl-1, 4-dihydroquinazolin-4-one (2) or 1-(1-benzyl-1, 2, 5, 6-tetrahydro-4-pyridyl)-2-methyl-1, 4-dihydroquinazolin-4-one (3), respectively. On the other hand, heating 4, the N₍₁₎-substituted derivative of 1d, with acetic anhydride gave 2-benzyl-5-methyl-1, 2, 3, 4, 5, 10-hexahydro-benzo[b]-1, 6-naphthyridin-10-one (5). The N₍₃₎-substituted derivatives (6 and 7) of 1d did not react with acetic anhydride.