Collagenolytic activity of rat kidneys with streptozotocin diabetes was estimated by means of a biological collagenase assay and compared to healthy controls. Collagenolytic activity was found significantly decreased in rat kidneys with diabetes correlating with blood glucose levels (r = -0.82, p < 0.001). Elevated blood glucose levels seem to be responsible for the inhibition. This is supported by our experiment of incubating bacterial collagenase with several carbohydrates as glucose, galactose and saccharose: glucose and galactose significantly inhibited the collagenolytic activity, while saccharose failed to inhibit the enzymatic reaction. The interpretation of the results is that glucose is able to bind to the enzyme as Schiff base, which could be shown by tritiated sodium borohydride reduction of the Schiff base formed between collagenase and glucose. Another support of the hypothesis is that blocking of the amino group of lysine at the active site either by glucose or trifluoroacetylation of collagenase is reducing the collagenolytic activity. The biological significance could be the decreased catabolism of collageneous material of the extracellular matrix, as, e.g., the glomerular basement membrane, which was reported in a previous publication.