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1. A voltage clamp has been developed for the pharyngeal muscle of the nematode Ascaris lumbricoides and has been used to analyse the potassium current that produces a negative-going, regenerative action potential in this muscle. 2. Depolarizing voltage steps elicit a sustained inward current; returning the membrane voltage to the resting level evokes a strong, transient, outward current. This outward current reverses direction at the same voltage as that reached by the negative-going spike and is identified as the negative spike current. 3. The negative spike current decays with a time constant of 30 msec at voltages more negative than -30mV. This inactivation of the negative spike conductance is removed by holding the membrane at potentials more positive than -15mV. The time constant for removal of inactivation decreases from more than 300 msec at -15 mV to about 30 msec at +10 mV. 4. When inactivation has been removed, the negative spike conductance is turned on by stepping to potentials more negative than -15 mV. 5. Although the reversal potential for this current depends strongly on [K+]o (42 mV/decade), the potential at which the conductance is turned on is independent of [K+]o. 6. External Na+ seems to facilitate the negative spike current. Reduction of [Na+]o reduces its conductance and shifts the reversal potential to more positive values. 7. External Rb+ and Cs+ show voltage-dependent blocking of this current. 8. This K current is different from all the K currents which have been studied previously; however, it is analogous to the classical Na current of nerve and muscle, except for an inversion of the voltage dependencies.
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