Freeze fracture of rat optic nerve reveals smooth, particle-free regions on the lammellar fracture faces of myelin when prepared by standard procedures. When the fixed, glycerin-impregnated tissue is incubated at 6 degrees C for two or more days, crystalline patterns indicative of a phase transition can be seen in the particle-free regions. The crystalline patterns can be destroyed by subsequent incubation at 37 degrees C and are not seen when the initial incubation is at room temperature or 37 degrees C. Butylated hydroxytoluene has no effect on the formation of the crystalline patterns. The time course of the formation of the crystalline patterns suggest that the rate-limiting step in the process is not the phase transition itself. We propose that the lipids associated with the particles in vivo are involved in the formation of the crystalline patterns.