A whole-cell early transient outward current occurs in rat myoballs if and only if there is an immediatly preceding current of large amplitude through the voltage-gated, tetrodotoxin-inhibitable Na+ channel. This early outward transient is a K+ current, designated IK(Na+). Under the conditions in which IK(Na+) appears, simultaneous measurement of voltage and current, under voltage clamp, demonstrates that there is transient voltage escape to depolarized levels, peaking at about the time of peak inward Na+ current arid resembling an action potential. IK(Na+) was never seen in the absence of this breach of the voltage clamp, suggesting that IK(Na+) might be an artefact due to transient depolarization from the clamp. However, when the voltage escape was mimicked by voltage commands under conditions in which the Na+ channel was not activated, there was no IK(Na). Insulin increased or produced IK(Na+) even though insulin had no effect on INa or on the delayed rectifier K+ current or on the escape from voltage clamp. It is concluded that there is a population of rat myoballs in which there is an early outward K+ current that requires an immediately preceding current through the voltagegated tetrodotoxin-inhibitable Na+ channel and is enhanced by insulin.