We have studied the strain-rate dependency of the viscoelastic mechanical properties of human dermis from young (23-year-old) and old (87-year-old) donors using incremental stress-strain measurements. The elastic spring constant for elastic fibers was found to be strain-rate and age dependent, whereas that for collagen was only age dependent. Fibril lengths were observed to decrease with increased strain rates and age for both elastic and collagen fibers; however, the large decrease in collagen fibril viscosity was hypothesized to be a result of thixotropy that results when neighboring collagen fibrils slide by each other. It is concluded that the elastic spring constant measured for elastic fibers may be higher than previously reported and is consistent with stretching of alpha-helical segments of elastin into a more extended conformation during the initial part of the elastic stress-strain curve. The decrease in the elastic spring constant with increased age observed is consistent with disruption of the elastic fibers and loss of alpha-helical structure. The pH dependency of the elastic modulus reported previously for collagen suggests that charge-charge interactions within and between collagen molecules are involved in energy storage during stretching. Elastic energy storage is consistent with the stretching of charged pairs located in flexible regions of the collagen molecule. Shear thinning, or thixotropy of skin, is hypothesized to reflect breakage of bonds that occur between collagen fibrils. It is hypothesized that both collagen and elastin are complex macromolecules that are hybrids of flexible and rigid regions. The flexible regions reversibly store elastic energy during stretching by breakage of secondary bonds. After stretching, the flexible regions become extended and transfer stress to the rigid regions of these molecules. This prevents premature mechanical failure of collagen and elastic fibers in the dermis.