Light-induced proton uptake in bovine retinal outer segment (ROS) fragments was shown to be closely related to pH, salt concentration, membrane integrity, and perhaps secondarily to the volume of osmotic compartments. The principal findings were as follows: 1. As pH increased, both the discs and the plasmalemma swelled, and proton uptake markedly diminished. 2. As the discs were disrupted by increasing concentrations of Triton, proton uptake at slightly alkaline pH was supplanted by proton release. 3. Increasing the concentration of chloride salts caused increased H+ uptake roughly proportional to osmotic shrinkage of the ROS. Buffering by acetate prevented the measurement of proton uptake in the presence of acetate salts, although osmotic behavior of the ROS was similar to that observed in chloride salts. Although increasing the concentration of sucrose also resulted in osmotic shrinkage of the ROS, it was not accompanied by a systematic increase in the magnitude of proton uptake. 4. Light-induced H+ uptake was accompanied by small but reproducible changes in volume, probably of the discs. The magnitude and direction of these rapid volume changes were subject to influence by pH, solute, and other variables.