For each of four rodent species, average weights for the brainstem, cerebellum, and forebrain were determined and average cross-sectional area of the cervical spinal cord (SCA) was calculated. Linear regression analyses against log body weight were performed on these data (log translated), along with data (except SCA) from the literature for insectivores and primates. Results indicate that the different CNS divisions scale by different powers of body weight in different mammals and that the rodent SCA varies by less than the 2/3 power of body weight. Based on the results, we conclude that there are at least two general brain scaling factors, somatic and nonsomatic, that necessitate a more complex general allometric equation; the exponent for somatic brain scaling is approximately 0.52, and body surface area is not a primary determinant of brain size.