PURPOSE: In the present study we have morphometrically examined a regeneration model in which axons normally residing in CNS have regrown and are interacting with Schwann cells from the PNS. This study will not only provide morphometric data on regenerated optic fibers but also shed light on possible factors in determining the fiber morphometry. METHODS: The optic nerves of rats aged 6 weeks were cut intra-orbitally and replaced with a autologous sciatic nerve. After a survival period of 9 months, the graft or "regenerated" nerves containing the regenerated optic axons and Schwann cells were processed for morphometric measurements. RESULTS: The mean myelinated axon diameter of regenerated nerve (1.8 +/- 0.2 micro m) was significantly (P < 0.05) greater than that of the optic nerve (0.9 +/- 0.03 micro m). However, unmyelinated regenerated optic axons had a smaller mean axon diameter (0.49 +/- 0.04 micro m) than normal myelinated optic axons. This may suggest that myelinating glial cells exert an influence on axon caliber and Schwann cells seem to have greater effect than oligodendrocytes. The mean g-ratio showing the relative myelin sheath thickness was found to be the highest in the optic nerve (0.78 +/- 0.003), least in the sciatic nerve (0.6 +/- 0.009) and intermediate in the regenerated nerve (0.68 +/- 0.01). The results indicated that Schwann cells myelinating the regenerated optic axons have produced a thinner myelin sheath. Intra-axonally, no significant difference was detected in the number of axonal microtubules and neurofilaments between the regenerated and optic nerves. Therefore the disposition of microtubules and neurofilaments into axon may be intrinsically determined. CONCLUSIONS: In this study, we have identified some of the extrinsic and intrinsic factors in determining the fiber morphometry of the regen-erated nerve. The axon-size and myelination by glial cells were determined through the external axon-glial interactions, whereas the number of axonal microtubules and neurofilaments were intrinsically determined.