1. A delayed current decrease associated with prolonged depolarization was studied in R(15) (the parabolic burster) of Aplysia by using intracellular recording and voltage clamp techniques.2. For long duration command pulses (3 sec), the outward current shows a delayed decrease. The current goes from a maximum near 100 msec and falls until a steady-state outward current is reached between 1.5 and 2.5 sec after the beginning of the command step. This final steady-state current is usually only about 20-30% of the peak outward current.3. Double step voltage clamps show that this current decrease is associated with a large shift of e.m.f. Measurements of conductance, on the other hand, fail to show any significant difference in conductance associated with peak and steady-state currents.4. From the results of application of high K(+) ringer, the conclusion is reached that this shift in e.m.f. is due to an accumulation of K(+) near the exterior cell membrane. Several other experiments exclude the possibility of either metabolic events or compensating conductance changes producing the phenomenon.5. The location of the accumulation is considered on the basis of anatomical studies. It is concluded that the accumulation takes place in the extensive infoldings found in cells like R(15). An explanation of the difference in delayed current decrease between pace-makers and non-pace-makers is suggested, since the pace-makers apparently have more extensive invaginations than the non-pace-makers. This suggestion is lent support by measurements of capitance and current density.