• Temperature measurements were taken from (1) the retina-choroid in the macula, (2) the scleral surface, or (3) the bulbar conjunctiva of the cynomolgus monkey, while the fellow eye was exposed to a moderate-intensity light source. Light stimulation produced an increase in tissue temperature in the nonlight-stimulated eye. The increase in tissue temperature presumably results from a reflexive increase in choroidal blood flow. Hydrogen washout measurements of blood flow in the retina-choroid confirmed this increase in flow. This active mechanism, along with the passive ability of the choroidal circulation to dissipate light-generated heat, may be an important physiologic safeguard in helping to maintain a stable temperature environment for the outer retinal layers in the macula.