The ability to understand the relationship between the reversal potential and the membrane potential is a fundamental skill that must be mastered by students studying membrane excitability. To clarify this relationship, we have reframed a classic experiment carried out by Hodgkin and Katz, where we compare graphically the membrane potential at three phases of the action potential (resting potential, action potential peak, and afterhyperpolarization) to reversal potential for K[superscript +] (E[subscript K]), reversal potential for Na (E[subscript Na]), and membrane potential (E[subscript m]) (calculated by the Goldman Hodgkin Katz equation) to illustrate that the membrane potential approaches the reversal potential of the ion to which it is most permeable at that instant.