The macromolecular structure of the axon membrane in optic nerves from 25-day-old male littermate control and myelin deficient (md) rats and 16-month-old md heterozygotic rats was examined with quantitative freeze-fracture electron microscopy. The axon membrane of control optic nerves displayed an asymmetrical partitioning of intramembranous particles (IMPs); P-fracture faces of myelinated internodal axon membrane were more particulate than those of pre-myelinated axons (∼1600 υ 1100 μm-2, respectively), while relatively few IMPs (∼150 μm-2) were present on external faces (E-faces) of internodal or pre-myelinated axon membrane. Amyelinated axons of md optic nerves also exhibited an asymmetrical partitioning of IMPs; protoplasmic membrane face (P-face) IMP densities, taken as a group, exhibited a wide range (∼600-2300 μm-2) and, in most regions, E-faces displayed a relatively low IMP density (∼175 μm-2). Axons of > 0.4 μm diameter exhibited significantly greater mean P-face IMP density than axons < 0.4 μm diameter. Aggregations of E-face IMPs (∼350 μm-2) were occasionally observed along amyelinated axon membrane from md optic nerves. Optic nerves from md heterozygote rats exhibit myelin mosaicism, permitting examination of myelinated and amyelinated axon membrane along the same tract. The axon membrane exhibits different ultrastructure in these two domains. Myelinated internodal axon membrane from md heterozygote optic nerves exhibits similar P- and E-face IMP densities to those of control internodal axolemma (∼1800 and 140 μm-2, respectively). Amyelinated axons in the heterozygote exhibit a membrane structure similar to amyelinated axons in md optic nerve. P-face IMP density of large diameter (> 0.4 μm) amyelinated axons from md heterozygote optic nerves is significantly greater than that of small calibre (< 0.4 μm) axons. In most regions, amyelinated axon membrane exhibits a relatively low E-face IMP density (∼200 μm-2); however, focal aggregations (∼400 μm-2) of E-face particles are present. Electrophysiological recordings demonstrate that amyelinated axons in md optic nerves support the conduction of action potentials. Compound action potentials in md optic nerves exhibit a monophasic configuration, even at 20-days postnatal, similar to that of pre-myelinated optic nerve of 7-day-old normal rats. Moreover, conduction velocities in the amyelinated 20-day-old md optic nerve are similar to those displayed by pre-myelinated axons from 7-day-old optic nerves. These results are consistent with persistence of action potential conduction in md axons, despite the absence of myelination in the optic nerves of the md mutant. © 1990 Chapman and Hall Ltd.