Abstract Hyperpiesin and miopiesin, pituitary principles affecting the intraocular tension, are shown to be distinct from vasopressin and oxytocin. Pitocin, which has chiefly oxytocin activity, has no effect on intraocular tension. Hyperpiesin and miopiesin are not found in the hypothalamus, whereas vasopressin is. Hyperpiesin and miopiesin are found in extracts of the posterior pituitary and spinal fluid. Only hyperpiesin is present in the anterior pituitary lobe. Hyperpiesin and miopiesin of rabbit pituitaries or Pitressin can be separated from vasopressin by countercurrent distribution between phosphate buffer of pH 6.0 and 1-butanol. In this distribution, a substance which converts hyperpiesin to miopiesin remains in the original tube, and the distribution constant for vasopressin is less than 0.25, and for hyperpiesin and miopiesin, about 3. The constant may be a little higher for hyperpiesin than for miopiesin.