This experiment was designed to evaluate the hue discriminative capacity of the guinea pig. Electrical observations revealed the presence of modulators in the retinas of these animals even though there are no cones. Four guinea pigs were trained to press a lever to obtain a food reinforcement, and a circle of light was projected upon the lever. The hue and luminance of this patch were varied with filters. In 2 animals, through differential reinforcement, a discrimination of brightness was established. Two other animals failed to discriminate red from green or green from blue. The data support the prediction of the duplicity theory that animals whose eyes contain rods alone are color-blind, and are not in accord with the contention that the guinea pig modulators are receptive elements for hue.