A computer model incorporating known behavior of the cardiovascular system and intravascular:interstitial fluid exchange was designed which allowed bleeding rate, IV infusion rate, and prehospital care times to be independently specified. All possible circumstances were examined. The model shows that IV's are potentially of benefit only when all of the following occur: 1) the bleeding rate is initially 25-100 ml/min, 2) the prehospital time exceeds 30 minutes, and 3) the IV infusion rate is approximately equal to the bleeding rate. IV infusions therefore appear of little benefit in the usual urban environment and have a sharply limited role overall. The possibility of pulmonary edema from fluid overload in nonhypovolemic patients, and reluctance of field personnel to infuse fluid at the rates necessary to produce benefit raise further questions about realistic benefit of IV's in all but the most rural systems.