The tensile strength of rat tail tendons, measured by the load that ruptures the tendon, varies with the weight of the animal, the vertebra to which the tendon was attached, the pH of the tendon, and the chemical treatment. Reduction with sodium borohydride multiplies the tensile strength by a factor up to 4.5 (the factor diminishing with age) and reduces creep in the strained tendon. It is concluded that strain catalyzes the hydrolysis of aldimine intermolecular crosslinks that contribute significantly to the tensile strength under physiological conditions, and that the crosslinks are stabilized by reduction or with aging. It is postulated that creep is the result of slippage between polymeric assemblies of stably crosslinked molecules. This postulate leads to a plausible description of elongation of connective tissue fibrils in growing animals.