Intracellular recordings from CA1 pyramidal cells were obtained in the in vitro hippocampal slice preparation before and after the application of morphine sulfate. Two distinct effects were observed following addition of morphine (1.0 mM) to the bathing media, or application to individual slices through pressure ejection via large tipped pipettes. Within 3-10 min following morphine application CA1 cell membrane potentials began to depolarize (5-10 mV) toward firing threshold. Spontaneous discharge rates were increased and nonactive cells became active. Rhythmic firing at a rate of 5-8 Hz was observed for 30-40 min following drug application. The spontaneous rhythmic firing could be partially reversed by the addition of naloxone (0.1 mM) to the bathing media. The second effect of morphine was to potentiate the development of epileptiform discharges in CA1 cells in response to low frequency orthodromic synaptic stimulation. The findings indicate that morphine alters the excitability of hippocampal pyramidal cells and such changes are likely to be mediated by opiate specific alterations in membrane conductance and/or tonic inhibitory synaptic inputs.