We loaded adult leopard frog optic nerves with the calcium-sensitive dye Calcium Green-1 3000 mw dextran conjugate. The dye was transported to the optic tectum in approximately 6 days and selectively labeled optic nerve terminals as seen with confocal microscopy. Viewed with an intensified CCD system, electrical stimulation of the optic nerve in vitro increases Calcium Green-1 fluorescence significantly. With increasing number of pulses in pulse trains there was increased presynaptic facilitation as measured by increased fluorescence. Addition of nicotine to the bathing solution increased baseline fluorescence. These results suggest that Calcium Green-1 dextran conjugate can be actively transported in adult nerve fibers over a significant distance and is retained in presynaptic terminals in a form that allows monitoring of presynaptic calcium levels.