Synchronous differentiation of delayed-rectifier potassium current regulates electrical excitability and calcium entry in motor, sensory, and interneurons of the developing amphibian spinal cord. Although Kv1 and Kv2 potassium channel transcripts are detectable in these cells, it is not known which transcript contributes to functional expression. Overexpression of a Kv1 dominant-negative subunit indicates that 20% of neurons have only Kv1 potassium currents. In other neurons, non-Kv1 channels function because the dominant-negative subunit either only partially suppresses or has no effect on current. Thus, diverse embryonic neurons coordinate differentiation of excitability yet rely on heterogeneous potassium channel gene expression.