A study of respiration and glycolysis in vitro of retinal pigment epithelium and retina minus the pigment epithelial layer has indicated metabolic similarities and differences between the two tissues. Aerobic lactic acid production from glucose was one-half in pigment epithelium what it was in retina, and the Pasteur effect operated with approximately the same efficiency in either tissue. Pigment epithelium, like retina, showed no evidence of an active phosphogluconate pathway under the experimental conditions used. On the other hand, despite evidence for a functional tricarboxylic acid cycle in pigment epithelium, glycolysis considerably predominated over oxidation in the catabolism of exogenous glucose by this tissue. Also different from what was found for retina was the observation that there was no CO 2 fixation in pigment epithelium.