Rats with implanted stimulating and recording microelectrodes were trained in a straight alley to repeatedly press a bar placed at one end of the alley and to run for water reward available at the other end. Stimulating the commissural input evoked field EPSP and population spike in the CA1 region while no population spike was observed in the dentate area. The amplitude of the CA1 population spike was maximum during running and smallest during drinking. Field potentials in the dentate gyrus changed in an opposite manner (drink greater than groom greater than press greater than run). Perforant path evoked cell discharges in the dentate gyrus which were maximal during drinking and smallest during running. The behavior dependent changes of the evoked potentials covaried with the frequency and power of the simultaneously recorded theta activity. Perforant path stimulation during the negative-going phase of the dentate theta cycle evoked significantly greater granule cell responses than stimuli during the positive-going phase. These observations suggest that the medical septum exerts a potent biasing effect on the efficacy of other afferent to the hippocampus.