Abstract: In physiological (10 mmol/L Ca2+)or low calcium saline (0.25 mmol/L Ca2+) bumps were evoked by dim continuous light (6×1069×106 photons/cm2; 10 s) in Limulus polyphemus ventral nerve photoreceptors.The photoreceptor was dark-adapted or weakly lightadapted by a 0.5 ms conditioning flash (9×109 photons/cm2),2.5 s prior to the continuous illumination.In 10 mmol/L external calcium the mean size (current-time integral)of the bumps was(39.5±1.76)pC; the light adaptation caused a decrease to (10.5±2.29)pC (n=4).In low calcium saline the effect of light adaptation was not reduced.The conditioning flash caused a reduction of the average bump size from (92±18.7)pC to (29±3.04)pC.However,when in low calcium saline the photoreceptors were stimulated by a few strong flashes (6.4×1014 photons/cm2) in order to exhaust the intracellular calcium stores,the effect of light adaptation by the conditioning flash was reduced.The light adaptation caused a reduction of the bump size from (81±10.1)pC to (41±3.74)pC.Injection of 1.2 mmol/L EGTA into the dark-adapted photoreceptors which were soaked by physiological saline almost abolished the effect of the light adaptation.Thus,we conclude that the illumination releases calcium from the organelles.The changes of the cytosolic calcium concentration decides the degree of the light adaptation of the photoreceptors.