Field potentials evoked by stimulation of afferent fibers in stratum radiatum were recorded in the CA1 region of the hippocampal slice maintained in vitro. Stimulation rates of 3-50/sec produced a large increase in amplitude of the population spike in CA1. This increase was maintained for several hours after the tetanization. The facilitation phenomenon appeared to be specific to the synapse of stratum radiatum afferents onto CA1 pyramidal cells since: (1) stimulation outside the radiatum layer did not produce the effect, (2) antidromic field potentials recorded in CA3 were unchanged, (3) EPSP threshold in CA1 was unchanged, and (4) alveus tetanization did not produce a facilitatory effect.