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 iv administration of PCr 500 mg·kg⁻¹ and 1 000 mg·kg⁻¹ the C-T curve could  be described by the two-compartment model with t_1/2β 22.5−23.3 min, V_d 0.956 4−0.978 6 L·kg⁻¹, CL 0.029 L·kg⁻¹·min⁻¹.  The Cr as PCr degraded product appeared as early as 2 min post iv dosing with t_max 20 min, t_{1/2k (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, t_{1/2k (m)} 56.0−57.7 min, metabolite-based bioavailability F_(m) 55.02%−62.31%.  PCr iv 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, t_1/2k 49−52 min.  In RBC no exogenous PCr was found but Cr was detected following iv administration of PCr, with the t_max 120 min and t_{1/2k (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 iv and po PCr.  Intravenous dosing leads apparently increased and sustained Cr and related-ATP concentration in RBC.