Calcium ions in the external medium stabilize the resting state of voltage-dependent channels, including Na channels. This effect of calcium on channel gating is usually explained in terms of the surface charge hypothesis, which proposes that local adsorption of calcium ion to the outside of the membrane alters the intramembranous electric field, thus influencing channel behavior indirectly. Calcium ion has also been shown to block Na channels, most strongly at negative voltage. We have examined these two apparently separate effects of calcium, the gating effect and Ca block, and find the two are closely correlated. We propose that calcium (or a suitable substitute) is an essential cofactor in normal gating and that it produces gating and blocking effects by binding within the channel.