The electrophysiological effects of phencyclidine (PCP) were measured intracellularly in guinea pig hippocampal CA1 neurons in vitro. At all doses tested (0.2 μM – 10 mM), PCP increased the width of action potentials (APs). Doses of 10 μM and higher were associated with decreased action potential amplitude. PCP decreased inhibitory postsynaptic potentials and excitatory postsynaptic potentials but did not alter responses to focally applied GABA. At the lowest dose (0.2 μM), PCP decreased the input resistance (R_in), while at all other doses R_in was increased. PCP decreased post-spike train afterhyperpolarizations at low and medium doses. PCP effects persisted in low calcium medium and also in medium containing 10⁻⁶ M tetrodotoxin. It is concluded that in these central neurons, PCP primarily blocks potassium conductances at all doses and, at anesthetic doses, depresses sodium-dependent spikes.Key words: phencyclidine, potassium conductance, CA1 neurons, electrophysiology.