Measurements of the instantaneous K current, under voltage clamp, were obtained using paired de- and hyperpolarizing clamp pulse sequences. The instantaneous potassium current-voltage relations so obtained showed constant field type rectification.2. The zero current potential of the instantaneous K current-voltage relation, when examined as a function of the K concentration, K, behaved like a Nernst concentration cell potential for K. It was concluded that during the generation of the delayed current, the membrane shows substantial selectivity for K.3. The equivalent of the ;Frankenhaeuser-Hodgkin space' in Myxicola was found to be more difficult to load with K ions than in squid. The thickness of the space may be taken as effectively about seven times larger than in squid.4. Myxicola axons were found to show little or no long time constant delayed current inactivation as has been described for squid.5. In high K a negative steady-state conductance was observed which was predictable from the independence principle with a suitable scaling factor.
