Incorporation of chelator into guinea‐pig rods shows that calcium mediates mammalian photoreceptor light adaptation.

1. The effects of steady light on the sensitivity and kinetics of the photocurrent response were studied in the rod photoreceptors of the guinea‐pig, using suction pipette recordings of circulating current. 2. The sensitivity of the flash response decreased with increasing background intensity according to Weber's law. Ultimately for the brightest backgrounds saturation ensued. The recovery phase of the flash response was accelerated by steady light, while the early rising phase was little affected. 3. These results indicate that guinea‐pig rods adapt to light in much the same way as do the rods and cones of lower vertebrates. 4. The role of cytoplasmic calcium concentration in this adaptation was studied by incorporation of the calcium chelator bis(o‐aminophenoxy)ethane‐ N,N,N',N'‐tetraacetic acid (BAPTA) into the rod cytoplasm. Superfusion with a solution containing the membrane‐permeant acetoxymethyl ester resulted in progressive changes in the response to light. 5. BAPTA incorporation retarded the falling phase of the flash response, thereby increasing receptor sensitivity, but did not affect the early rising phase of the response. BAPTA also slowed the adaptation of the response to steady illumination. 6. These results indicate that cytoplasmic calcium concentration plays a similar role in the light adaptation of guinea‐pig rods to that in the adaptation of the rods and cones of lower vertebrates. Calcium therefore appears to act as the messenger of light adaptation in mammalian rods.