Purpose Several studies have shown strong correlations between myelin content and T1 within the brain, and have even suggested that T1 can be used to estimate myelin content. However, other micro‐anatomical features such as compartment size are known to affect longitudinal relaxation rates, similar to compartment size effects in porous media. Methods T1 measurements were compared with measured or otherwise published axon size measurements in white matter tracts of the rat spinal cord, rat brain, and human brain. Results In both ex vivo and in vivo studies, correlations were present between the relaxation rate 1/T1 and axon size across regions of rat spinal cord with nearly equal myelin content. Conclusion While myelination is likely the dominant determinant of T1 in white matter, variations in white matter microstructure, independent of myelin volume fraction, may also be reflected in T1 differences between regions or subjects. Magn Reson Med 75:1341–1345, 2016. © 2015 Wiley Periodicals, Inc.