The slow excitatory postsynaptic potential (EPSP) was investigated in frog sympathetic ganglion cells. In contrast to the increased conductance associated with other known EPSP's, during the slow EPSP resting membrane conductance was decreased. Electrical depolarization of the membrane potentiated the slow EPSP, whereas progressive hyperpolarization decreased its size and then reversed it to a hyperpolarizing potential (the opposite of the effect of membrane polarization on other EPSP's). The reversal potential of the slow EPSP was close to the potassium equilibrium potential. We propose that the slow EPSP, in contrast to classical EPSP's, is generated by an inactivation of resting potassium conductance.