Using the pulse-labeling method, the rate of the slow component (SC) of axonal transport was analyzed during maturation and aging. Ventral motor neurons and retinal ganglion cells of 3-, 6-, and 24-month-old Fischer 344 rats were radiolabeled with 35S-methionine. To measure the rates of SCa and SCb subcomponents, distributions of the total radiolabeled proteins and certain cytoskeletal proteins (actin, clathrin, tubulin, and the neurofilament proteins) were analyzed in the ventral root-sciatic nerve and optic nerve. Our results show that the rate of transport for both SCa and SCb proteins decreases with age in ventral motor axons and optic axons. For example, in ventral motor axons the rates of both SCa and SCb decreased 40% between 6 and 24 months. These results, with those of others, show that the rate of slow transport gradually decreases in the neurons of adult rats (7,11) The factors that may contribute to the slowing are discussed.