Reissner's fibre was investigated in brains and cervical spinal cords of mice aged 28 and 31 months using scanning electron microscopy. At both 28 and 31 months Reissner's fibre was present. It arose from a fusing together of numerous smaller fibres in the subcommissural organ and was observed in the aqueduct, fourth ventricle and central canal of the medulla and spinal cord. For most of its length it was a ridged structure similar to a stick of celery but in places small processes could be seen arising from its surface. Cilia, supraependymal cells and other debris appeared to be adherent to the fibre and this may support the view that Reissner's fibre has a detoxifying function which is still present in old age.