Single neurone recordings from primate hippocampal and parahippocampal areas were made during the performance of a sensory-motor association task. Responses of neurones were analysed for one pair of stimuli to which the monkey had learned to make particular arm movements. A single association was found to be coded by between 2.2 and 7.2% of the population of neurones, depending on the particular region sampled. Neurones in the subicular complex and from the CA3 subfield had twice the probability of activation of those neurones from TF-TH and the CA1 subfield. Regional variation was also found for the distribution of differential response latencies. These results are discussed in relation to neural models of memory storage and retrieval, and suggest that a given learned association is coded by a higher proportion of neurones within the hippocampal system than was predicted on theoretical grounds alone.