Activation of Ca channels has been described as a Hodgkin-Huxley process in which the time constants have ratios of two or greater (1, 2, 3) or as a three-state linear sequential process with two very different time constants (4). We examined activation by measuring the turn-on and turn-off of macroscopic Ca currents and the discharge of single Ca channels. The results show that neither a literal Hodgkin-Huxley mechanism nor a three-state model applies. A minimum model of activation requires at least four states.