Influence of hydrogen-bond-breaking reagents such as urea on shrinkage temperature, isometric tension, swelling behavior, tensile strength, and percentage extension of native rat tail tendon (RTT) were examined. The swelling behavior was observed with polarizing optical microscopy and scanning electron microscopy. The results show that the lyotropic swelling increased the width of the fiber and was associated with the action of urea on the collagen fiber. Hydration properties led to significant variations in the swelling phenomenon. Lyotropic swelling produced opaque, limp, and flaccid fibers that did not change appreciably in length. The melting behavior and the swollen fascicles were clearly seen in scanning electron micrographs of 3 and 6M urea-treated RTT. The reduction in the dimensional stability of native RTT collagen fiber on treatment with urea demonstrated the role of secondary structure in the dimensional stabilization of collagen.