The role of viscoelasticity of collagen fibers in bovine articular cartilage was examined in compression and tension using stress relaxation measurements in the axial direction (normal to the articular surface). Experimentally, for a given axial strain, both peak and equilibrium loads were higher in tension than in compression, whereas stress relaxation was stronger in compression, as indicated by the higher peak-to-equilibrium ratios. A viscoelastic fibril-reinforced model including fluid flow was used for analysis of the experimental data. The collagen fibrillar matrix was assumed to be viscoelastic with a strain-dependent tensile modulus, and the nonfibrillar matrix was modeled as linearly elastic. For axial tension, collagen viscoelasticity was found to account for most of the stress relaxation, while the effects of fluid pressurization on the tensile stress were negligible. In contrast, for axial compression, the dominant mechanism for stress relaxation arose from fluid pressurization, while the associated relaxation in collagen fibers mainly resulted in an increase in radial strain. The effective Poisson's ratio, defined as the ratio of the radial and axial strains, was generally smaller in compression than in tension, and deviated from the true Poisson's ratio in tensile tests because of the frictional contacts between the specimen and the loading platens. Furthermore, lower collagen elasticity in the axial direction was observed than in the radial direction. This study illustrates the essential role of collagen viscoelasticity and interstitial fluid pressurization in the mechanical response of articular cartilage.