This article is report the study of the pharmacokinetics and metabolic disposition of exogenous phosphocreatine (PCr) in rats by means of an ion-pair HPLC-UV assay. PCr and its metabolite creatine (Cr) and related-ATP in rat plasma and red blood cell (RBC) were simultaneously determined. A blank plasma and RBC were initially run for baseline subtraction. Plasma and RBC samples were deproteinized with 6% PCA prior to HPLC. Following i.v. administration of PCr 500 mg x kg(-1) and 1 000 mg x kg(-1) the C-T curve could be described by the two-compartment model with t1/2beta 22.5-23.3 min, V(d) 0.956 4-0.978 6 L x kg(-1), CL 0.029 L. kg(-1) x min(-1). The Cr as PCr degraded product appeared as early as 2 min post i.v. dosing with t(max) 20 min, t1/2kappa (m) 40.6-42.7 min and f(m) 60%-76%. After po administration of PCr, the parent drug in plasma was undetectable, but the metabolite Cr was detected with t(max) 65-95 min, t1/2kappa (m) 56.0-57.7 min, metabolite-based bioavailability F(m) 55.02%-62.31%. PCr i.v. administration resulted in significant elevation of ATP level in RBC but not in plasma, the related-ATP in RBC was characterized by t(max) 68-83 min, t1/2kappa 49-52 min. In RBC no exogenous PCr was found but Cr was detected following i.v. administration of PCr, with the t(max) 120 min and t1/2k (m) 70 min for Cr. The above results indicate that PCr eliminates and bio-transforms in body very rapidly; K > K(m) confers ERL, instead of FRL, type upon the metabolic disposition of Cr. Following po administration of PCr, the degraded product Cr is absorbed but not the parent drug PCr. The formed Cr can be accounted for by most of i.v. and po PCr. Intravenous dosing leads apparently increased and sustained Cr and related-ATP concentration in RBC.