The rheological behavior of mammalian tendon is analyzed in terms of its constituents structure and their properties. The elastic fibers are assumed to be straight and linearly elastic. They are of predominant role in the low ranges of strain. The collagen fibers are nonuniformly undulated. Upon stretch they gradually become straight, thus increasing the stiffness of the tissue. They are assumed to be linearly viscoelastic with negligible bending strength. It is shown that the nonuniformity of the collagen fiber structure can account for the observed nonlinear load-strain relations as well as for the nonlinear viscoelastic behavior of the tendon. An experimental procedure is developed through which the material functions and parameters can be determined.