We present systematic measurements of longitudinal relaxation rates (1 / T 1) of spin polarization in the ground state of the nitrogen-vacancy (NV-) color center in synthetic diamond as a function of NV- concentration and magnetic field B. NV- centers were created by irradiating a Type 1b single-crystal diamond along the [100] axis with 200 keV electrons from a transmission electron microscope with varying doses to achieve spots of different NV- center concentrations. Values of (1 / T 1) were measured for each spot as a function of B.