Differential scanning calorimetry (DSC) has been applied to the study of connective tissue to evaluate the denaturation process of collagen. We have applied this technique to the study of the ageing of rat skin. We have tried to correlate the variations of the parameters measured by DSC and the modifications of collagen crosslinks with ageing. The thermograms obtained are composed of one main peak located between two shoulders. The relative size of each peak varies with time: the first peak diminishes regularly from 2 to 20 months whilst, at the same time, the third peak increases; the recovery temperature increases with age (+ 16 degrees C between 2 and 20 months); the total denaturation enthalpy does not vary: the main value obtained is 5.9 X 10(-2) J/mg collagen. On the other hand, the assay of reducible collagen crosslinks in rat skin, over the same age range, shows a decrease of heat-labile aldimine crosslink (essentially hydroxylysinonorleucine). These results and the study of thermograms obtained with altered rat skin (animals treated with beta-aminopropionitrile or skin reduced with NaBH4) allow us to conclude that heat-labile and heat-stable crosslinks account for a collagen thermal stabilization which can explain the delay of denaturation characterized by the third peak of DSC thermograms.