Membrane depolarization relieves the G protein-mediated inhibition or block of high threshold Ca2+ channel currents. We found that the net rate of reblocking depended on the extent of G protein activation. With low intracellular concentrations of GTP gamma S reblocking rates resembled inactivation rates; with higher concentrations reblocking rates increased progressively. Reblocking kinetics were fit with a sum of two exponential functions having time constants (in ms) tau F greater than or equal to 10 and tau S greater than or equal to 30. Unblock during depolarization was fit by a single exponential function with time constant tau A similar to tau F. A model was developed in which unblocking followed dissociation of a blocking molecule, possibly the G protein itself, from Ca2+ channels, and reblocking occurred at rates that depended on the concentration of the blocking molecule. The time course of Ca2+ entry and thus presynaptic Ca2+ levels can be regulated by both the concentration of the G-protein-dependent blocking particle and membrane potential.