A voltage-gated hydrogen ion-selective conductance has been previously described in the immature oocyte of the urodele amphibian Ambystoma. The present study was prompted by reports that changes in membrane voltage and internal pH, as well as in internal sodium ion concentration, occur during the hormone-induced maturation of oocytes from other amphibians. As activation of membrane currents might mediate changes in internal ion concentrations in addition to altering the membrane voltage, microelectrode recording techniques have been employed to examine changes in membrane conductances which occur during maturation of Ambystoma oocytes. It was observed that during the first 5 hr of maturation the magnitude of the hydrogen ion conductance gradually decreased, and that subsequently there was an increase in the amplitude of a voltage-dependent noninactivating sodium conductance. After 6 to 7 hr, after the loss of the hydrogen conductance and at about the time of germinal vesicle breakdown, the resting potential of the oocyte spontaneously shifted from approximately -10 mV to approximately +30 mV, where it remained until at least 24 hr after the initiation of maturation. This voltage transition was due to the appearance of mechanisms generating inward current in the oocyte membrane; part of this inward current was due to the tonic activation of the sodium conductance. Changes in internal pH and internal sodium ion concentration occurred during maturation, as judged from shifts in the reversal potentials of both hydrogen and sodium currents. A gradual decrease in internal hydrogen ion concentration was observed up until the time of disappearance of the hydrogen conductance (change in internal pH from about 7.15 in immature oocytes to about 7.40 by 3 hr after application of progesterone). This was followed, as sodium conductance increased, by an apparent rise in the internal sodium ion concentration (from about 6 mM to about 17 mM by 10 hr postprogesterone).