Under acidic and mild-temperature conditions, 1-methyl-2-phenylindole was found to react with malondialdehyde (MDA) and 4-hydroxyalkenals to yield a stable chromophore with intense maximal absorbance at 586 nm. The use of methanesulfonic acid results in optimal yields of chromophore produced from MDA as well as from 4-hydroxynonenal. By contrast, the use of hydrochloric acid results in an optimal yield of chromophore produced from MDA and a negligible reaction of 4-hydroxynonenal. Taking advantage of such chromogenic reactions, we developed a new colorimetric assay of lipid peroxidation. Using a methanesulfonic acid-based medium, MDA and 4-hydroxyalkenals can be measured at the 586 nm wavelength. However, the presence of endogenous inhibitors of the reaction with 4-hydroxyalkenals is common, and this means that the latter may be underestimated in some biological samples. The assay performed in a hydrochloric acid-based medium enables the specific measurement of MDA in the presence of 4-hydroxyalkenals. Upon hydrolysis of Schiff bases in hydrochloric acid (pH 1.5), either assay can be used to specifically measure the amount of total MDA in biological samples because 4-hydroxyalkenals undergo an irreversible cyclization reaction under the hydrochloric acid-based conditions of hydrolysis. The two assays were applied to the determination of the amount of MDA alone and of MDA and 4-hydroxyalkenals in an in vitro model of lipid peroxidation. This methodology was also used to clarify complex patterns of tissue-specific MDA production in vivo, following hydrolysis of Schiff bases, in rodents treated with doxorubicin.