The macroscopic rate constants of formation (k(1)) and hydrolysis (k(2)) for the reactions of pyridoxal 5'-phosphate (PLP) with hydrazine (PLP-HY system), carbidopa (alpha-hydrazino-alpha-methyl-beta-(3,4-dihydroxyphenyl)propionic acid, acid, PLP-CD system), hydralazine (1-hydrazinophthalazine, PLP-HL system) and isoniazid (4-pyridinecarboxylic acid hydrazide, PLP-ISO system) were fitted to a kinetic scheme that considers the different ionic species present in the medium, their protonation constants, and their individual rates of formation (k(1)(i)) and hydrolysis (k(2)(i)). The results obtained for the molecules bearing the hydrazine group are compared with those for the reactions of PLP with n-hexylamine (PLP-NHA system) and poly-L-lysine (PLP-LYS system). Some structural effects on the individual rate constants are also examined.