Short-term synaptic plasticity was studied in the in vitro hippocampus of the North American opossum (Didelphis virginiana) and rat (Rattus norvegicus). Conditioning and test stimulus pulses were delivered to fibers in stratum radiatum, and intracellular and extracellular recordings were obtained from area CA1 pyramidal cells. In rat, the amplitude of the population spike in response to the second (test) of two stimulus impulses is suppressed at short inter-pulse-intervals (IPI's). In opossum, the amplitude of the test population spike is facilitated at comparable IPI's. Facilitation of the test population spike in rat occurs only when the test stimulus is separated from the first stimulus (conditioning) by a longer IPI. Peak values of facilitation do not significantly differ between species. Intracellular responses, elicited by stimulus pulses that were subthreshold for spike production, indicate that the amplitude of test EPSP's recorded from opossum pyramidal cells are facilitated at IPI's that result in suppression of test EPSP's in rat pyramidal cells.