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A comparative neurophysiological study of the reactivity of hippocampal EEG in dog, cat and human (an epileptic patient with chronically indwelling electrodes) was carried out. In all 3 species the same type of qualitative changes in hippocampal spectral parameters in the theta frequency band (peak frequency, peak amplitude and rhythmicity) in relation to behavioural tasks was encountered. These results were obtained by means of computing triggered averages of spectral parameters in relation to behavioural events. Both in dog and in cat it was found that transitions in behaviour from a low intensity (e.g. sitting) to a high intensity (e.g. walking) motor activity were accompanied by an increase in the value of the afore mentioned spectral parameters. In man a similar relation between such a transition in gross motor behaviour and hippocampal EEG was, however, not observed. The highest frequencies in the theta band appeared in man to be related to "verbal behaviour and writing". In all 3 species the following generalization appeared to hold; when the same behavioural state occurs at different levels of intensity, a higher intensity of the behaviour--as measured in terms of its motor components--is related to higher values for the spectral parameters of the hippocampal EEG.