Thermal stability measured by isometric contraction-relaxation force was examined in rat tail tendons after incubation in vitro in glucose or hydroxymethylfurfurale solutions at pH 7.4 using buffer systems of either phosphate or tris (hydroxymethyl)aminomethan. In the phosphate system, incubation with glucose (170 mmol/l) for 12 days was found to increase the thermal stability of the tendons by a factor 3. At the same time, glucose was found to be attached to the lysine and hydroxylysine residues of collagen, and reactive carbonyl compounds were formed in the solution. In the tris(hydroxymethyl)aminomethan system containing reactive amino groups (pK 8.1), glucose was also attached to the lysine and hydroxylysine residues, but only very small amounts of reactive carbonyl compounds were formed in the solutions and no changes in thermal stability were recorded. Incubation with hydroxymethylfurfurale itself was found to increase the thermal stability rapidly and markedly in the phosphate buffer systems. This effect was inhibited when the tris(hydroxymethyl)aminomethan buffer system was used. Buffer solutions with tris(hydroxymethyl)aminomethan, containing large amounts of free amino groups compared to the free amino groups of collagen, might interfere with the formation of cross-links formed by carbonyl groups derived from metabolic glucose and amino groups of collagen. The non-enzymatic glycosylation of lysine and hydroxylysine itself does not influence the thermal stability. Additional reactions appeared to be transformation into reactive carbonyl compounds, such as hydroxymethylfurfurale, with subsequent formation of thermally stable cross-links between the collagen molecules.