Isoplumbagin (isoPLB, 5-hydroxy-3-methyl-1,4-naphthoquinone), a naturally occurring quinone, has been observed to exercise anti-inflammatory, antimicrobial, and antineoplastic activities. Notably, whether and how isoPLB, plumbagin (PLB), or other related compounds impact transmembrane ionic currents is not entirely clear. In this study, during GH-cell exposure to isoPLB, the peak and sustained components of an ( related gene)-mediated K current () evoked with long-lasting-step hyperpolarization were concentration-dependently decreased, with a concomitant increase in the decaying time constant of the deactivating current. The presence of isoPLB led to a differential reduction in the peak and sustained components of deactivating with effective IC values of 18.3 and 2.4 μM, respectively, while the value according to the minimum binding scheme was estimated to be 2.58 μM. Inhibition by isoPLB of was not reversed by diazoxide; however, further addition of isoPLB, during the continued exposure to 4,4'-dithiopyridine, did not suppress further. The recovery of by a two-step voltage pulse with a geometric progression was slowed in the presence of isoPLB, and the decaying rate of activated by the envelope-of-tail method was increased in its presence. The strength of the hysteresis in response to an inverted isosceles-triangular ramp pulse was diminished by adding isoPLB. A mild inhibition of the delayed-rectifier K current () produced by the presence of isoPLB was seen in GH cells, while minimal changes in the magnitude of the voltage-gated Na current were demonstrated in its presence. Moreover, the identified in MA-10 Leydig tumor cells was blocked by adding isoPLB. Therefore, the effects of isoPLB or PLB on ionic currents (e.g., and ) demonstrated herein would be upstream of our previously reported perturbations on mitochondrial morphogenesis or respiration. Taken together, the perturbations of ionic currents by isoPLB or PLB demonstrated herein are likely to contribute to the underlying mechanism through which they, or other structurally similar compounds, result in adjustments in the functional activities of different neoplastic cells (e.g., GH and MA-10 cells), presuming that similar in vivo observations occur.