AbstractIn RB cells ofAplysia, serotonin, in the presence of TEA, 4AP and Ba, elicits a voltage‐dependent inward current. In Ba‐TEA‐4AP seawater, RB cells showed a negative slope region (NSR) in their current‐voltage (I‐V) relationship when measured at the end of 2‐s commands from a holding potential of −60 mV. Addition of serotonin to the bathing solution enhanced the NSR. When holding potential was lowered to −10 mV, the NSR as well as the effects of serotonin were greatly reduced. Addition of 20 mMcobalt to the bathing solution blocked both the NSR and the inward current produced by serotonin. Changes in potassium concentration produced no consistent shift in voltage sensitivity nor change in amplitude of the current elicited by serotonin. Intracellular injection of cesium sufficient to broaden action potentials did not block the enhancement of NSR by serotonin. These results support the conclusion that in RB cells, serotonin produces a voltage‐dependent current carrie