In tissue fixed with glutaraldehyde, dissolved O2 is rapidly consumed by two processes: residual respiration and glutaraldehyde-induced chemical uptake. The nature of the chemistry which consumes O2 during tissue fixation was investigated by studying model reactions of glutaraldehyde with amines and with homogenized tissue suspensions. The addition of glutaraldehyde to solutions of most primary amines and ammonia stimulated rapid O2 consumption. The reaction of glutaraldehyde with primary amines (eg, 25 mM ethanolamine, glycine, or methylamine) consumed 50% of the dissolved O2 in 15 to 20 s at 37 degrees C. The initial rate of O2 uptake followed second-order kinetics with respect to the primary amine concentration. The total amount of O2 consumed was sufficient to account for the stoichiometric conversion of the primary amines to pyridines. These data are consistent with the synthesis of pyridine derivatives from glutaraldehyde-amine precursors in which the last step is an irreversible oxidation of dihydropyridines to pyridines. The addition of glutaraldehyde to homogenized muscle suspensions, in which respiration was chemically inhibited, significantly increased the rate of O2 uptake. Thus, in tissue O2 is rapidly depleted both by respiration and the chemical demands of the glutaraldehyde-amine reactions during the cross-linking process. Since these experiments were done under conditions commonly used for tissue fixation, hypoxia should be assumed to exist in biological preparations fixed with glutaraldehyde.