Although ATP-MgCl2 administration after hemorrhage and resuscitation restores the decreased hepatic blood flow, it is not known whether this is due to the increase in portal blood flow or hepatic arterial blood flow. To study this, rats underwent a midline laparotomy (i.e., trauma induced) and were bled to and maintained at a mean arterial pressure of 40 mmHg until 40% of maximal shed blood volume was returned in the form of Ringer lactate (RL). The animals were resuscitated with four times the volume of the shed blood with RL, during and after which ATP-MgCl2 (50 mumol/kg body wt) or an equal volume of normal saline was infused intravenously over 95 min. Cardiac output and organ blood flow were determined by 85Sr-labeled microspheres at 90 min after the completion of resuscitation. The results indicate that portal blood flow and total hepatic blood flow decreased significantly after hemorrhage and resuscitation. ATP-MgCl2 treatment, however, restored these parameters to sham values. In contrast, hepatic arterial blood flow did not change significantly after either hemorrhage and resuscitation or ATP-MgCl2 infusion. Moreover, the depressed cardiac output was normalized and coronary blood flow was higher than shams after ATP-MgCl2 treatment. Unlike small intestinal blood flow, blood flows to the stomach, spleen, pancreas, mesentery, and cecum were not markedly affected with ATP-MgCl2 infusion. Thus the restoration of hepatic blood flow with ATP-MgCl2 treatment under such conditions is due to the increased portal blood flow, i.e., solely due to the increased small intestinal blood flow.