We have examined the development of an inactivating outward current in embryonic amphibian neurons differentiating in culture. On the basis of ionic selectivity, voltage dependence of activation and inactivation and pharmacological sensitivity, it is similar to A currents described in other neurons. In these embryonic neurons the A current appears later than other voltage-dependent currents studied previously. Furthermore, its maturation extends to times later than those required for the differentiation of another potassium current, the voltage- dependent delayed rectifier. These changes in A current are consistent with parallel changes in the action potential and excitability of the developing neurons.