Temporal summation of monosynaptic EPSPs produced by paired afferent stimuli was studied in the experiments on identified neurons of mollusc Planorbis corneus. Nonlinear properties of the summation were shown; there were three types of the summation: facilitation, depression and mixed type. It was shown that summation functions (characterizing the dependence of the ratio between the test and conditioning EPSP amplitudes and the value of the corresponding interstimulus++ interval) had some local extremum in most cases. The value of the interstimulus++ interval corresponding to this extremum was in correlation with single EPSP parameters. Possible localization of mechanisms responsible for the EPSP summation properties is discussed.