Abstract The density of microtubules and the axonal caliber were studied in myelinated axons from sural nerves of young male alloxan-diabetic and age-matched rats, at 0, 15, 30, and 60 days after diabetes induction. The longitudinal growth of axons was normal in diabetic rats, but in contrast, the radial growth of axons was impaired. The axonal area was 20% less than normal in diabetic rats at the 60th day. Microtubule density was assessed in 3-μm-diameter axons. No differences in microtubule density (range: 25.0 to 28.0 microtubules/μm 2) were found between or within diabetic and control groups. However, the subnormal axonal size in diabetic rats entailed a reduced content of microtubules in a nerve trunk because the relation between axonal size and microtubule density remained normal. The results showed that microtubule density was independent of axonal length, and of age, weight, or diabetic conditions of the rats, and was related only to the axonal size. These findings have implications in nerve cell homeostasis.