1. The effects of extracellular H+ (pHo) in the pathophysiological range (pH 6-8) on voltage-gated sodium, potassium, and calcium currents were examined in acutely dissociated rat hippocampal CA1 neurons using the whole-cell patch clamp technique. All experiments were conducted in Hepes-buffered solutions and were performed at room temperature (21-23 degrees C). 2. TTX-sensitive sodium currents, evoked by both step and ramp depolarization, were reversibly depressed by moderate acidosis and enhanced slightly by alkaline exposure. Changes in current amplitude were coincident with small reversible shifts (+/- 3 mV) in the voltage dependence of activation. In contrast, sodium current activation and decay kinetics as well as steady-state inactivation were unaffected by acidosis. 3. Outward potassium currents could be separated into a transient, rapidly inactivating current (IA) and a sustained, slowly inactivating component (IK). Steady-state activation of both currents was unaffected by an increase or decrease in pHo. Similarly, IK activation and IA decay kinetics remained stable during pHo exchange. In contrast, the steady-state inactivation (h infinity) of both potassium currents was reversibly shifted by approximately +10 mV during acid exposure, but remained unchanged during alkaline treatment. 4. Calcium currents were found to be predominantly of the high-voltage-activated (HVA) type, which could be carried by Ba2+ and inhibited completely by cadmium. Moderate acidosis (pH 6.9-6.0) reversibly depressed HVA Ca2+ current amplitude and caused a positive shift in its voltage dependence. For both of these parameters, alkaline treatment (pH 8.0) had the opposite effect. The depression of HVA Ca2+ currents by low pHo was unaffected by raising the internal Hepes concentration from 10 to 50 mM in the patch pipette. A Hill plot of the effect of pH on Ca2+ current amplitude revealed a pK value (defined as the mid-point of the titration curve) of 7.1 and a slope of 0.6. 5. The rate of Ca2+ current activation was unaffected by pHo at positive potentials, but below 0 mV the activation rate increased at low pH and decreased at high pH, becoming significant at -20 mV. At this membrane voltage, a second HVA current was revealed during acid exposure as the whole-cell HVA current was depressed. Ca2+ current decay was described by two time constants, both of which were significantly reduced at pH 6.4 and slightly enhanced at pH 8.0. Steady-state Ca2+ current inactivation reached 50% near -50 mV and was not affected at either pH extreme. 6. These results demonstrate that extracellular pH shifts within the pathophysiological range are capable of modulating both the conductance and gating properties of voltage-gated ion channels in hippocampal CA1 neurons. The effects we describe are consistent with the wellknown effects of pHo on neuronal excitability and strengthen the idea that endogenous pHo shifts may help regulate cell activity in situ.