Membrane currents in the immature oocyte of the urodele amphibian Ambystoma were studied using the two-micro-electrode voltage-clamp technique. A current carried by H ions (IH) constituted the major portion of outward current activated by depolarizations from the resting voltage (about -60 mV). Net inward current was not observed at this developmental stage. The reversal potential for IH measured from tail currents obtained in two step voltage-clamp experiments shifted by 54 mV per unit change in external pH between pH 6.9 and 8.4. The reversal potential at pH 7.4 was not affected by changes in external K or Cl concentrations. A small change in reversal potential was observed with removal of external Na. The amplitude of IH was not affected by removal of external Ca (Mg or Sr substitution). Ca ionophore A23187 shifted the current-voltage relation towards negative voltages. Activation of IH did not appear to depend on Ca influx. The instantaneous current-voltage relation for IH, measured from tail currents at approximately equal internal and external H ion concentrations, was linear between -40 and +30 mV. The steady-state conductance-voltage relationship was sigmoidal with membrane voltage, and, at pH 7.4, was one-half maximum at about +15 mV (V1/2). The time courses of activation and deactivation were proportional to 1-exp (-t/tau). A plot of time constant (tau) against voltage was bell-shaped, with a maximum near V1/2. These results suggested that the activation of IH is voltage dependent. Increases in the external H ion concentration shifted the conductance-voltage and time constant-voltage relations in parallel towards positive voltages. The magnitude of these shifts showed a lower saturation near pH 9. Low concentrations of external Cd (10-300 microM) reduced current amplitude by shifting the current-voltage relation in the positive direction. Cd also reduced the limiting slope conductance. These effects were partially reversible.