Simultaneous measurements of load, deformation and diameter were performed on stretched collagen fiber bundle from rat tail tendon using a dynamic, electronically controlled stretch system and a novel computer based electroptical set-up. A parallel analysis of glycosaminoglycan (GAG) concentration in the bathing solution was carried out to determine whether stretching affects GAG exudation from the bundle. Results show that the bundle diameter does change under stretch in a manner which depends on strain and time. The diameter decreases with time under constant axial strain, implying loss of fluid from the structure. Results of GAG analysis showed that stretching accelerate their exucation to the external bath. The data from cyclic stretch tests show that low (0.5%) strain produces monotonically decreasing diameter from cycle to cycle. Yet at higher strain level (4%) under sufficiently long rest periods between cycles, the diameter increases monotonically with cycling to above its original level, implying damage to restraining elements in the bundle which maintain its structural integrity. Simultaneous load and diameter data show mutually different trends, indicating that variation in the bundle's hydration (diameter) in itself does not have a significant effect on the bundle's axial response.