2-Methyl-4-amino-6-hydroxypyrimidine (I) undergoes Mannich reaction with formaldehyde and piperidine, even in aqueous alkali solution and forms 2-methyl-4-amino-5-piperidinomethyl-6-hydroxypyrimidine (II), but the use of an excess of formaldehyde and piperidine results in the formation of the bis (piperidinomethyl) compound (III) of (I). Examination of the chemical properties and ultraviolet absorption spectrum of the bis compound (III) indicated the compound to be 2-methyl-4-piperidinomethylamino-5-piperidinomethyl-1, 6-dihydropyrimid-6-one (IIIf) and, at the same time, (I) and (II) were assumed to be the 1, 6-dihydropyrimid-6-one compounds (Ia) and (II′). 2-Methyl-4-methylamino-1, 6-dihydropyrimid-6-one (VII), similar to the 4-amino compound (Ia), also undergoes Mannich reaction with formaldehyde and piperidine to form 2-methyl-4-methylamino-5-piperidinomethyl-1, 6-dihydropyrimid-6-one (VIII).