Osmolal, electrolyte, and other responses in peripheral blood were recorded after water loading of patients with ocular hypertension and chronic simple glaucoma, and with a small group of younger normal subjects. The osmolal responses of the ocular hypertensive group were significantly greater than for the glaucoma group, and further differences in plasma potassium concentration were also found. Drinking habits may exert a significant influence in ocular hypertension. The relative contributions to the diagnostic category of the variables recorded was evaluated by discriminant analysis, and the application of this retrospective approach to the assessment of new patients is discussed.