A quantitative method is described for the determination of formaldehyde in biological tissues by stable isotope dilution using gas chromatography/mass spectrometry. (13C2H2)Formaldehyde is used as the isotopic diluent. After tissue homogenization, derivatization is carried out in situ with pentafluorophenylhydrazine, followed by extraction and analysis using selected ion monitoring. The sensitivity of the technique is higher than that of conventional methods of formaldehyde analysis, enabling endogenous formaldehyde to be quantitatively analyzed in tissues, even in samples as small as 20 mg wet weight. The effects of exposure to airborne formaldehyde or to airborne methyl chloride on the formaldehyde concentrations of several tissues of Fischer-344 rats are reported.