Modulation of the electrophysiological properties (amplitude, latency, paired-pulse facilitation) of the CA<sub>1</sub> hippocampal field by different concentrations of RU 486 was studied in hippocampal slice preparations from normal (sham) and adrenalectomized BALB/c mice. In the sham group as well as in the adrenalectomized group, RU 486 had no effect with the lower concentrations used (0.05 and 0.5 n M), but produced a decrease in the population spike amplitude for the higher concentrations used (5 and 50 n M). This decrease was delayed and sustained. Combined application of RU 486 at 5 n M and corticosterone at 0.5 or 5 nM displayed an excitatory effect, but no inhibitory effect. Combined application of RU 486 and dexamethasone showed no significant effect. These results are related to the antagonist and agonist properties of RU 486 and to the different steroid receptor systems in the hippocampus.