Development of dose dependent chronic irreversible cardiotoxicity is a key problem encountered in chemotherapy with adriamycin. Here it has been demonstrated that infusion of this agent produced distinct and largely irreversible changes in levels of phosphate metabolites and substantial acidosis that are detected by 31P NMR of the Langendorf perfused heart. Administration of the antioxidant, butylated hydroxytoluene minimizes these spectral changes but does not substantially diminish the antineoplastic activity of adriamycin. Bisantrene (CL 216,942), a noncardiotoxic anthracene with antineoplastic activity, produces only minor perturbations of the 31P spectrum of the perfused rat heart. These studies demonstrate the potential utility of employing 31P NMR to monitor acute or chronic cardiotoxicity in the perfused rat heart and for developing noninvasive in vivo NMR techniques for monitoring cardiotoxicity in experimental animals and humans.