THE oxidative metabolism of the ciliary body has been investigated with regard to the energy source for the secretion of aqueous humour. It has been shown previously that formation of aqueous humour is dependent on active secretion of Na+ (ref. 1) and there is some evidence that this process is effected by means of a sodium-potassium stimulated ATPase2-4, a finding in keeping with results on other secretory tissues5,6. The source of ATP or other high-energy intermediates utilized in Na+ secretion must be from glycolysis or the citric acid cycle, as de Roetth7 has shown that the oxidized substrates of ciliary body are chiefly carbohydrate. Cole8 has shown that certain enzymes of the citric acid cycle are present in the epithelial cells and that succinic dehydrogenase is concentrated inthe non-pigmented layer.