Illumination of white-eyed Musca photoreceptors following hypoxia or the application of ruthenium red (RR, a known blocker of Ca2+ uptake into intracellular organelles) induced a transient after depolarization (TA). The TA was enhanced when external [Ca2+] was reduced; it was abolished when external [Na+] was reduced to a level that affected the receptor potential to a small degree. The TA was enhanced or depressed when the activity of Na/K pump, which controls the Na+ gradient, was enhanced or depressed respectively. This effect was observed even when the receptor potential was not affected. All of the above observations are consistent with the hypothesis that the TA is triggered by a light-induced increase in the concentration of intracellular free Ca2+ which appear to be very high, following treatments with hypoxia or RR. The high sensitivity of the TA to Na+ and Ca2+ gradients across the photoreceptors membrane strongly suggests that the TA is due to a transient activation of an electrogenic Na-Ca exchange mechanism which depolarizes the cell.