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Senile muscle atrophy has been attributed to an impaired ability of old nerves to transport trophic factors. To evaluate the effect of age on axoplasmic transport, we measured the accumulation of cholinesterase activity above a ligature around sciatic nerves of young (7--8 months), middle-aged (19--20 months), and old (31--32 months) male rats. Protein content and cholinesterase activity per mm of nerve were higher in middle-aged and old than in young nerves. However, accumulation of cholinesterase activity was significantly lower by middle age and was strikingly reduced by old age. This large reduction in axoplasmic transport appeared to result from factors other than axonal loss. A model in which old nerves have an increased number of temporary focal blockages of particle movement in axoplasmic channels is proposed to explain the decreased transport.