To a very good first approximation, CA1 field potentials (1Hz -10KHz; X1,000) evoked in freely moving rats by microstimulation of CA3 conform closely to expectations from hippocampal slices and from the hippocampus of anesthetized rats (Hargreaves & Muller, SFN 2000). Thus, in CA1 str. oriens we saw a positive going 3 msec event corresponding to the population EPSP from pyramidal cells. The amplitude of this event grew with increasing stimulus intensity; at higher intensities a biphasic, initially negative pyramidal cell population spike was superimposed on the field EPSP. With electrodes in and deep to str. pyramidale the sign of the population EPSP but not the population spike was inverted. In addition to the usual evoked field potential we saw in some rats a short latency (1 - 1.5 msec) negativity whose sign did not reverse as the electrodes were driven into str. radiatum. This early event occurred at lower stimulus intensities than later pyramidal cell population events. We suggest that the early event...