Abstract: We studied the spatial summation of excitatory post synaptic potentials (EPSPs) at proximal and distal dendrites of the hippocampal CA1 area by using two stimulating electrodes in anaesthetized Wistar rats. Our results showed that the summation of EPSP was sublinear both at the proximal and the distal dendrites of hippocampal CA1 area. As expected, EPSP amplitudes increased and the summation of distal dendritic EPSPs decreased (though tended to be more sublinear) but the proximal summation was not affected. Furthermore, bicuculline (GABA receptor antagonist) didnt affect the spatial summation of EPSPs, but AP-4 (antagonist of transient A type K+ channel) made spatial summation of the proximal dendrites linear. Our experiments demonstrate that hippocampal CA1 pyramidal neurons may use different patterns of spatial summation at proximal and distal dendrites. Proximal dendrites of hippocampal CA1 pyramidal neurons receive mainly the inputs from schaffer collateral/associational commissural pathways, whereas the distal dendrites receive mainly the inputs from entorhinal cortex through TA (temporoammonic) pathways. These results indicate that that two different summation mechanisms in hippocampal CA1 pyramidal neurons are used to process information (1) within the hippocampus and (2) from the entorhrinal cortex.