The thermal stability of collagen fibres from rat tail tendon was studied in both normal and diabetic rats. Results were correlated with the age of the animals, glycosylated haemoglobin level and the degree of non-enzymatic glycosylation of the collagen. Age was found to be the most important single determinant of the thermal rupture time of collagen fibres (r = 0,87, p less than 0.005). Thermal rupture time was also increased in diabetes and showed good correlation with glycosylated haemoglobin (r = 0.69, p less than 0.005) and non-enzymatic glycosylation of collagen (r = 0.51, p less than 0.005). The correlation of glycosylated haemoglobin level and thermal rupture time was significant, independent of age and non-enzymatic glycosylation of collagen. Experiments in vitro showed that incubation of collagen fibres with glucose increased the thermal stability of collagen. These results demonstrated an independent effect of severity of diabetes on the physical properties of collagen. The role of non-enzymatic glycosylation in causing the changes in collagen properties in diabetes remains to be fully elucidated.