(1) Sex and sex steroids are potent modulators of membrane excitability. (2) Within the components of the action potential, sex steroids are known to affect subthreshold responses, action potential duration and the after-hyperpolarization (Figs. 2-4). (3) The ion channels underlying these events of the action potential are direct sites of action for both the acute and chronic effects of sex steroids. (4) Both estrogen and testosterone can stimulate Ca2+-activated K+ channels and normal estrogen function is important to normal K+ channel expression. (5) Estrogen and testosterone have been shown to modulate voltage-gated Ca2+ channel function and expression.