To evaluate the effects of phosphocreatine (PCr) and creatine (Cr) depletion on skeletal muscles of mice deficient in muscle creatine kinase (M-CK), we have fed mutant mice a diet containing the creatine analogue beta-guanidinopropionic acid (beta GPA). After 8-10 weeks of feeding, accumulation of the creatine analogue in M-CK-deficient muscles was comparable to that observed in muscles of wild-type mice. Strikingly, and unlike wild types, mutants did not accumulate phosphorylated beta GPA, indicating that MM-CK is the only muscle CK isoform which can phosphorylate beta GPA. In M-CK-deficient muscles there was respective depletion of PCr, Cr and ATP levels to 31, 41 and 83\% of normal. The average cross-sectional area of type 2B fibres in gastrocnemius muscles was very much reduced and was similar to type 1 and type 2A fibres which maintained their normal size. The maximal isometric twitch force developed by gastrocnemius-plantaris-soleus (GPS) muscle complexes of beta GPA-treated mutants was reduced by about 30\%, but these muscles showed an increased fatigue resistance during 1 and 5 Hz contraction. Mitochondrial enzyme activities in the upper hind limb musculature of null mutants were 20-35\% increased by the beta GPA diet. Altogether, these results provide evidence that certain functions of the creatine kinase/phosphocreatine (CK/PCr) system are not eliminated solely by the loss of M-CK.