A transversely isotropic biphasic mixture model was applied to tendon in uniaxial tension. Parametric analyses were performed and the sensitivity in predicting material parameters was evaluated. Our results provide quantitative evidence for fluid flow as a mechanism that contributes to tendon viscoelasticity. Transversely isotropic material properties were calculated for mouse tail tendon fascicles. The average transverse modulus (E₁) was 0.046 MPa, the fiber-aligned Poisson's ratio (ν₃₁) was 2.73, and the transverse Poisson's ratio (ν₂₁) was 0.96;​ these properties were not strain-dependent. The fiber-aligned modulus (E₃) was strain-dependent and was 20.7 MPa in the toe region and 86.1 MPa in the linear region. These solid matrix properties were consistent with previously published tendon tissue and fascicle data. The fascicle permeability was strain-dependent and was 5.5×10⁻¹⁸ m⁴/N s in the toe region and 0.32×10⁻¹⁸ m⁴/N s in the linear region, similar to previously reported meniscus permeability in tension. The similar permeabilities of both fascicle and tissue-level samples suggest that fluid flow from individual fascicles, not the packing of multiple fascicles together, may be the primary barrier to fluid flow in tendon and thus the primary mechanism for viscoelasticity.