In order to investigate the responses of neurites to myelin in vitro, retinal explants obtained from adult rats were cultured on poly-L-lysine and laminin. Regenerating adult ganglion cell axons were confronted with rat central myelin which was adsorbed on culture dishes. When the explants were placed directly on the myelin, axons did not grow out. Alternatively, when the explants were placed at a certain distance from the myelin, massive outgrowth of axons was observed. When axonal growth cones encountered the myelin, they displayed various reactions. All single axons observed immediately stopped growing after encountering the myelin border. All of them remained for at least 4 h at the border. Sixteen per cent of them remained in this 'resting state' throughout the time of observation (2 - 5 days). Nineteen per cent of the axons degenerated following contact with the myelin. The majority of axons were able to circumvent the myelin either by backbranching (21%) or by growing in another direction with the leading growth cone (37%). Eight per cent of the axons were observed to enter the myelin. The results indicate that central myelin represents a non-permissive obstacle for growing adult axons. This observation is in conformity with the inability to regenerate that is typical of lesioned optic nerve axons in situ.