We assessed the effect of streptozotocin-diabetes on in vivo collagen metabolism in skin, aorta and intestine by injecting [ 3H]proline into rats, 20 days after administering the diabetogen, streptozotocin. One day after [ 3H]proline injection, diabetic and control animals were killed, their tissues analyzed for both 3H-labeled and unlabeled hydroxyproline and results expressed per entire tissue. Thereby, the effect of diabetes on net collagen synthesis and tissue collagen mass, respectively, was evaluated. Diabetes resulted in a lower content of [ 3H]collagen in skin and aorta, suggesting decreased net collagen synthesis. This decrease in net synthesis was accompanied by a decrease of collagen mass in skin, whereas aortic collagen mass was unaffected. Consequently, an acceleration of collagen degradation in skin is postulated to have accompanied the expected depression of collagen synthesis; alterations of the physiochemical properties of skin from diabetic rats support this interpretation. For intestine, both net collagen synthesis and mass increased in diabetic rats, reflecting increased collagen synthesis—possibly associated with polyphagy. In conclusion, with regard to collagen metabolism, representative connective tissues respond differently to experimental diabetes, and we suggest that this insight will be useful in future studies aimed at understanding the pathophysiology of connective tissues affected by diabetes.