The summation of excitatory postsynaptic potentials (EPSPs) generated in separate parts of the dendritic tree of hippocampal pyramidal cells has been investigated using the in vitro slice preparation. Two separate inputs with known synaptic location were used. The EPSP produced by simultaneous activation of the two inputs (observed sum) was compared to the algebraic sum of the individual EPSPs. Small-amplitude EPSPs (0.5-1.5 mV) added linearly. The shortest distance between the two synaptic groups was 75 micron. With larger amplitudes (greater than 2.5 mV), the EPSP summated nonlinearly. The nonlinear summation was reduced by moderate hyperpolarizations (2-10 mV) of the soma membrane. Also, large EPSPs (greater than 2.5 mV) summated linearly when the peak of the summed EPSP was brought close to the equilibrium potential for the inhibitory postsynaptic potential (IPSP) (EIPSP). When the EPSP peak was made more negative than the EIPSP, summation was again nonlinear but the algebraic sum was now smaller than the observed EPSP sum, i.e., the direction of the nonlinearity was reversed. EPSP summation was linear after the IPSP had been blocked by benzyl penicillin application. We conclude that separate EPSPs in hippocampal pyramids (minimal separation, 75 micron) add linearly but that the addition of an IPSP may complicate this picture. No evidence was found for interaction between the different populations of excitatory synapses.