High T1-weighted signal appears in brain of patients who have experienced MRI examinations using gadolinium contrast agent several times. The relaxation effect of gadolinium contrast agent strongly depends on the viscosity of the solution, and the average viscosity in the cell is higher than that of the free water. However, the relaxation effect of the gadolinium contrast agent in the cell has been unknown. In this study, we investigated the longitudinal relaxivity of it – concentration dependence of longitudinal relaxation rate in solution with intracellular viscosity. The intracellular viscosity strongly enhances the longitudinal relaxivity of the gadolinium contrast agent.