Following intravenous administration of 32P-phosphocreatine (50 mg/kg) or an equimolar mixture of creatine + 32P-phosphate (equivalent to 50 mg/kg phosphocreatine) to rats, and examination of an extract of cardiac muscle by HPLC, the proportion of radiolabelled adenosine triphosphate (ATP; up to 3%) was similar in each treatment group at 120 min after dosing. Mean concentrations of ATP in the cardiac muscle were increased significantly during this time (to 3.1 mumol/g) after intravenous administration of 32P-phosphocreatine compared with those in control rats (2.5 mumol/g) and in rats dosed with creatine + 32P-phosphate mixture (2.6 mumol/g). Phosphocreatine concentrations in cardiac muscle of phosphocreatine-treated rats were also increased (2.3 mumol/g) significantly after 120 min compared with controls (1.7 mumol/g), but were unchanged after 30 min. Mean concentrations of phosphocreatine in rats receiving a creatine + phosphate mixture were slightly reduced (1.3 and 1.5 mumol/g after 30 min or 120 min, respectively) compared with controls. The elevation of tissue ATP and phosphocreatine levels may be involved in the protective effect provided by exogenous phosphocreatine against anoxia in isolated cardiac muscle.