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1. The membrane mechanism of a quasi-stable membrane depolarization (latch-up) that persists in darkness following red light was examined in barnacle photoreceptor with micro-electrode techniques including voltage-clamp and Na+-sensitive micro-electrodes. 2. Current-voltage (I-V) relations of the membrane in darkness following red light (latch-up) and in darkness following termination of latch-up with green light, indicate that latch-up is associated with an increase of membrane conductance. 3. The latch-current (membrane current in darkness following red light minus membrane current in darkness following a gree flash that terminates latch-up) was inward at the resting potential, reversed sign at about +26mV (mean of six cells), and became outward at more positive membrance potentials. 4. Current-voltage relations of the membrane during green light (no latch-up) closely resembled those during latch-up. The light-induced current (LIC) elicited by green ligh (membrane current during the light flash minus membrane current in darkness following the light flash) was inward from the resting potential to +26mV (mean of six cells), then reversed sign and became outward. 5. The latch-current and LIC were both augmented in reduced Ca2+ solutions and decreased as Na-+ was reduced at a fixed Ca2+ concentration. 6. Both LIC and latch-current reversed sign at a more negative membrane potential (increment V equals 14mV) in solutions containing one quarter the normal amount of Na+. 7. The internal Na-+ activity (a-iNa) of a photoreceptor increased from about 10-18 mM upon illumination with long steps of intense red or white illumination. Five minutes in darkness after white light, a-iNa had recovered significantly, whereas a-iNa remained elecated following red illumination. 8. Latch-up seems to be a persistence in darkness of the same membrane mechanism that normally occurs during illumination; i.e. a conductance increase to Na+ ions. Ca2+ ions act primarily to suppress this current. There is evidence for a net Na+ influx during illumination that is sustained in darkness during latch-up.