The reactivity of formaldehyde (FA), acetaldehyde (AA) and propanal (PA) towards malondialdehyde (MDA) under physiological conditions was investigated. These alkanals revealed a reactivity towards MDA. Especially, FA showed high reactivity; for instance, a mixture of MDA (1 mM) and FA (6 mM) incubated for 4h, or a mixture of MDA (1 mM) and FA (1 mM) incubated for 24 h, resulted in almost a 100% decrease of MDA. The amount of free MDA decreased as the alkanal concentration increased and in the reaction of MDA with AA or PA, the loss of MDA was in approximate agreement with the yield of 2,4-dihydroxymethylene-3-alkylglutaraldehyde (1b or 1c). In the reaction of MDA with FA, the yield of 1a decreased with an excess amount of FA. In a thiobarbituric acid (TBA) test, 1 gave a positive reaction. Compounds 1b and 1c, which contain 2 molecules of MDA, liberated 1 molecule of MDA during the TBA reaction. The results showed that the MDA recovery of 1b and 1c with the TBA test was 50%. In the case of 1a, MDA recovery was 32%. In the MDA/p-nitrophenylhydrazine (NPH)-HPLC method, the MDA recovery of 1a, 1b and 1c ranged from 3-7%.