The increase in acid-insoluble collagen (AIC) from tail tendons of streptozotocin-diabetic rats was measured and compared with that for control rats. AIC increased from 10% initially to 75% after 12 weeks of diabetes. It then increased slowly to 85% after 45 weeks. AIC for control rats was constant for the first 12 weeks and then increased slowly to 40% after 45 weeks. These data are consistent with an increase in the number of acid-stable cross-links in the collagen due to diabetes. The quantity of collagen solubilized by pepsin at 4 degrees C was unaltered due to diabetes, strong evidence that formation of diabetes-induced cross-links between helical regions of collagen molecules cannot explain the increase in AIC observed. Non-enzymatic glycation (NEG) increased linearly over 45 weeks, but the rate of NEG was much slower than the rate of increase in AIC observed for diabetics. The level of NEG for diabetics was about three times that for controls at a given time, but there was still less than 1 mol of glucose detected/mol of collagen at near maximum acid insolubility. Fluorescence associated with tail tendons was measured to test the hypothesis that fluorescent cross-links form as a consequence of NEG and result in decreased collagen solubility. Fluorescence (lambda ex 370; lambda em 430) increased slowly with age but was similar for control and diabetic tendons of the same age. Fluorescence was not increased in AIC compared with acid-soluble collagen derived from a given tendon sample. NEG of collagen reached near-diabetic levels in non-diabetic rats whose growth was inhibited by restricted feeding, but there was no associated increase in AIC. These data suggest that NEG and the subsequent formation of fluorescent cross-links do not contribute significantly to the rapid increase in AIC in the streptozotocin-rat model of diabetes.