Calcium trapped within sonicated and resealed bovine rod outer segment disks is released upon light exposure with a stoichiometry of 0.75 +/- 0.05 calcium for each rhodopsin bleached. The amount of calcium liberated is proportional to the amount of bleaching in the range of 20 to 100% bleaching and is relatively insensitive to the internal trapped calcium concentration. The results are obtained using a flow system in which the disk membrane vesicles are adsorbed on glass particle supported by a filter. The external calcium is washed away and subsequent calcium release is monitored by collecting fractions of the effluent before, during, and after light exposure. Disks that are sonicated and allowed to reseal prior to incubation with 45Ca show no change in calcium efflux upon bleaching. The light-activated calcium release is also eliminated if disks sonicated in the presence of 45Ca are treated with a calcium ionophore prior to bleaching. The results demonstrate that the light-released calcium comes from the disks and not from the external disk surface. Lowering temperature to 3--4 degrees C surpresses the light-stimulated release, implicating a transition after the formation of metarhodopsin I in the transport process. The resluts suggest a model for the disk in which each bleached rhodopsin functions as a "one-shot carrier" to transport a single calcium ion across the membrane.