Cytosolic phosphocreatine spontaneously hydrolyzes to yield creatinine and orthophosphate (Borsook and Dubnoff 1947). Creatinine cannot be metabolized further and is excreted from the body in the urine. Creatinine formation proceeds at a nearly constant rate and the amount produced by an individual is a function of muscle mass, so urinary creatinine output is clinically useful as a normalization factor in assays of urinary output of other molecules. Iyengar et al. (1985) have suggested that an alternative reaction sequence, proceeding via phosphocreatinine but also spontaneous, may contribute to creatinine formation.