Phosphocreatinine undergoes the following spontaneous simultaneous reactions at pH 7.4 (0.02 M sodium phosphate and 120 mM KCl) and 38 degrees C. (Formula: see text) The first order rate constants are 0.046 h-1 (ka) and 0.048 h-1 (kb). There is a major effect of pH on the reactions such that at pH values higher than 7.4 phosphocreatine production predominates, while at pH values less than 7.4 creatinine is the major product. This along with titration data showing apparent pK values of about 3.0 and 7.5 for phosphocreatinine suggest that the dianionic form of phosphocreatinine is involved in the conversion to phosphocreatine, whereas the monoanionic form is exclusively converted to creatinine. Possible mechanisms to account for the reactivity of phosphocreatinine are discussed. Several lines of evidence suggest that the apparent Keq for phosphocreatine formation from phosphocreatinine is about 300 at pH 9.0 and about 70 at pH 7.0, and the delta G0' (pH 7.0) is-2.6 kcal/mol. The delta G0' (pH 7.0) for the hydrolysis of the phosphoryl bond in phosphocreatinine is-12.8 kcal/mol. The phosphocreatinine content of rabbit white skeletal muscle was measured to be 0.05 mumol/g, which is 0.4% of the phosphocreatine content. The in vitro experiments suggest that phosphohydrolysis of phosphocreatinine can account for a creatinine formation equal to 0.5% of the phosphocreatine content/day. We conclude that it is likely that a substantial fraction of the in vivo creatinine production from phosphocreatine goes through the novel high energy phosphate, phosphocreatinine, as an intermediate.