In an epileptic patient with depth electrodes placed in the hippocampal formation we observed the following: (1) There existed a significant peak in the theta band of the hippocampal EEG of this subject under a number of behavioural circumstances. (2) Four different behaviours were performed at two degrees of intensity. Whenever the values of a certain spectral parameter associated with a low or a high intensity behaviour showed a significant difference, the higher value corresponded with the more intense version of the behaviour. (3) The frequency and rhythmicity of the hippocampal theta component during writing was consistently higher than during sitting or walking. The amplitude was smaller. (4) In a word association task the amplitude, frequency and rhythmicity showed a significant rise during the period of silence immediately following the question and preceding the answer.