because the rate of equilibration of ammonia between body fluids is un¬ known. It has been estimated that 7.2 gm of urea per day is converted to ammonia in the gastrointestinal tract of adults.13 Thus, our estimate of am¬ monia turnover may be low by about 40%. If this is the case, an even smaller percentage of daily ammonia production would be removed by an exchange transfusion. We have also assumed that the blood used for the exchange has a normal ammonia con¬ centration. This may not be the case, since the ammonia concentration rises in shed blood.