Although it has been shown that infusion of adenosine triphosphate (ATP)-magnesium chloride (MgCl2) proved beneficial in the treatment of shock, it is not known whether this effect is due to improvement in the microcirculation or to direct provision of energy. In searching for the mechanism of this, we have now examined the in vitro uptake of ATP by soleus muscle of animals in shock. Rats were bled to a mean arterial pressure of 40 mm. Hg and so maintained for 2 hours. Following death the two soleus muscles from each animal were removed and incubated in Krebs-HCO3 buffer containing 10 mAA. of glucose, 5 mAA. (8-14C) of ATP, 5 mM. (8-14C) of ADP, or 0.5 mM. (8-14C) of adenosine, and 5 mM. of MgCl2 for 1 hour under an atmosphere of 95 percent O2 to 5 percent CO2. Following homogenization and centrifugation, samples of the muscle extract and the medium were subjected to electrophoresis to separate the various nucleotides. The concentrations of the several nucleotides in medium and muscle were calculated from the radioactivity observed in each fraction. The uptake of 14C-ATP by muscles from animals in shock was three times greater than was the uptake by control muscles. This leads us to conclude that the beneficial effect of ATP-MgCl2 to animals in shock could be due to provision of energy directly to tissues in which ATP levels were lowered. © 1975.