Summary When the isolated guinea pig heart is perfused with C14- and P32-labeled ATP of known isotope ratio, ATP isolated from the myocardium shows an isotope ratio 100 times greater than the starting material. Inorganic P32 is liberated into the perfusion fluid along with C14-labeled dephosphorylation and deamination products of ATP. UTP, GTP, CTP and ITP are also dephosphorylated when perfused. The results indicate that the ATP molecule does not enter the myocardium as such, but first is degraded and later resynthesized intracellularly.