A rise in serum citrate will usually be accompanied by a fall in pH during massive transfusion of citrated blood. Inorganic phosphate will rise. Total Ca may remain at normal or near normal levels, but calculated ionized Ca falls. Serum K is usually within normal limits, but may rise markedly during exchange transfusion. If transfusion is administered during general anesthesia, these effects may be potentiated by the myocardial depressant effects of the anesthetic agent. If refrigerated blood is administered rapidly, the resultant hypothermia may further modify these effects. Any harmful metabolic effects of massive transfusion are almost certainly the result of the interaction of many of these simultaneous individual effects. Prevention of possible metabolic effects of transfusion can be achieved, at least under elective conditions, by the use of fresh heparinized blood, and this remains the preparation of choice for cardiopulmonary bypass and for exchange transfusion in newborn infants. However, when large amounts of blood are needed for emergency transfusion, we are limited to the use of blood as commonly collected and stored in acid citrate dextrose (ACD) solution. In spite of the rather dramatic chemical alterations which occur in ACD blood, it remains by far the most serviceable preparation available. Efforts to correct the "chemical lesion" of collection and storage of blood in ACD solution should be viewed with some caution. The "corrected" blood may be more harmful than the unmodified blood. On the other hand, the possible metabolic effects of massive transfusion should be borne in mind and appropriate diagnostic and therapeutic measures taken.