We investigated the microscopic structure of transverse sections of the oculomotor, trochlear and abducens nerves of Arabian foals using stereological methods. Bilateral nerve pairs from 2month-old female Arabian foals were analyzed. The tissues were embedded in plastic blocks, then 1 mu m thick sections were cut and stained with osmium tetroxide and methylene blue-azure II. Stereology was performed using light microscopy. Morphometry showed that the right and left pairs of nerves were similar. The transverse sectional areas of the oculomotor, trochlear and abducens nerves were 1.93 +/- 0.19 mm(2), 0.32 +/- 0.06 mm(2) and 0.70 +/- 0.08 mm(2), respectively. The oculomotor nerve exhibited a significantly greater number of myelinated axons (16755 +/- 1279) and trochlear (2656 +/- 494) and the abducens nerves (4468 +/- 447). The ratio of the axon diameter to myelinated nerve fiber diameter was 0.58, 0.55 and 0.55 for the oculomotor, trochlear and abducens nerves, respectively. Of the three nerves studied, the abducens nerve exhibited the greatest nerve fiber area, myelin area, nerve and axon diameters, and myelin thickness. The ratio of small myelinated nerve fibers was greatest in the oculomotor nerve.