The effect of protein malnutrition on collagen crosslinking was studied in different age groups of rats. The percent of reversibility of neutral salt-soluble collagen gel and the susceptibility of insoluble collagen to denaturing agents and pronase decreased with aging. The electrophoretic pattern of neutral salt-soluble collagen on sodium dodecyl sulphate polyacrylamide gels showed a decrease of alpha 1 and alpha 2 subunits and an increase of beta-components, resulting in a decrease of alpha/beta ratio in aging. There was a significant decrease in the aldehyde content of neutral salt-soluble collagen during age. Protein deficiency was found to impair the crosslinking and the effect was greater in younger animals than in older ones.