Functional regulation of potassium currents in developing neurons is pivotal for changes in excitability and action potential waveform. Here, we test whether an excess of potassium channel transcripts is sufficient to drive functional expression of potassium current and shortening of the duration of the action potential. Injection of Shaker- like potassium channel transcripts into two-cell stage embryos achieves increases in RNA levels. The elevated levels of potassium channel RNA produce larger delayed-rectifier currents. Action potential durations are briefer, indicating that larger potassium currents are not compensated by changes in inward currents. Strikingly, overexpression of potassium current RNA leads to a reduction in the number of morphologically differentiated neurons in culture. We suggest that, by prematurely reducing the duration of the impulse, early overexpression of potassium channel activity suppresses normal developmental cues.