Specific cortical somatosensory evoked responses were recorded in nine patients suffering from senile dementia and eight elderly depressed patients. The results were as follows. (1) The latencies of each peak were longer in the demented than in the depressed group but this difference was significant only for peak 3 (the second negative deviation). (2) The late negative deviation (peak 5) was either absent or very flat in demented patients. The ratio of the amplitude of this late wave to that of the first peak discriminated between demented and non-demented patients at the 0·5% level. (3) The value of this ratio combined with that of the latency of peak 3 could be used to classify all but one patient into the correct diagnostic group. The results are discussed in the light of the relevant literature. It is suggested that slowing of conduction in the nervous system may be a very basic disturbance associated with cognitive impairment in old age and that measurements of this kind may come to assist in the diagnosis of dementia.