Calibres and microtubule contents of the non-medullated and myelinated domains of optic nerve axons of adult rats were studied with the electron microscope. The cross-sectional areas of the non-medullated domain was 0.25 microm2, and that of the myelinated domain 0.40 microm2, that is, greater by 59%. The increase in size was uneven across the axonal population; it was marked in fine and medium sized axons, and modest in the largest axons. The number of microtubules increased with axonal size; the density, however, decreased from 85 mirotubules/microm2 in 0.1 microm2 axons to about 20 in 1.2 microm2 axons. In axons of equal cross sectional area, the microtubular density of the myelinated and non-medullated domains was the same. Microtubular density values of optic axons resemble those of dorsal roots more than those of peripheral nerve axons of equal calibre. The facts that optic axons increase in size and gain microtubules behind the eyeball while the microtubular packing decreases suggest a local regulation of the axonal cytoskeleton.