I_slow is not due to a Na/Ca exchanger. To evaluate the possibility that the slow current may reflect the activity of an electrogenic Na/Ca exchanger triggered by the light-evoked increase in cytosolic Ca²⁺, cells were stimulated with bright flashes while superfused with solution containing no sodium. The slow current was still observed, regardless of the replacement of extracellular sodium. With lithium, which permeates readily through the light-dependent ion channels, a large photocurrent (truncated in the figure) was also elicited. During superfusion with guanidinium or NMDG, in contrast, the initial inward current was severely reduced (<100 pA), as the photoconductance is poorly permeable to these ions; nonetheless, the subsequent slow current developed normally, both in terms of time course and amplitude. Light intensity of 1.6 × 10¹⁵ photons × cm⁻² × s⁻¹; holding potential of −60 mV.