Results of 58 biaxial and uniaxial stress-relaxation tests on flat specimens of abdominal rabbit skin are analyzed. It is shown that rabbit skin is a nonlinear viscoelastic material, i.e., the relaxation mode of each stress component depends on the strain (or alternatively on the initial stress). The dependence of the relaxation mode on the initial stress is investigated in terms of a modified power law with three parameters (Eq. (15)) which are functions of the initial stress. The differences and similarities between uniaxial and biaxial stress relaxation tests and the effect of the tissue “preconditioning” is discussed. On the basis of the results a theory is developed for the uniaxial viscoelastic behavior of rabbit skin tissue when small incremental strains are super-imposed on large constant deformation.