For over one hundred years speculation has occurred in regard to the permeability of the bladder wall to physiologic as well as foreign substances. The reason for again taking up the subject is that a possible clinical application arose that seemed to warrant investigation. In a patient of this clinic, following an injury that amounted to transection of the cervical cord a marked cystitis developed, a complication that persisted in spite of cystotomy and generous lavage of the bladder. With this, there developed an abnormally high nonprotein nitrogen content of the blood, though a high fluid intake and output had been maintained, and though the function of the kidneys as measured by phenolsulphonphthalein excretion was good. In a conversation with Dr. Frank Fremont-Smith of the Harvard Neurological Unit, the possibility of reabsorption from the bladder was suggested as responsible for this turn of events, and the inquiry outlined here was