Summary.The paper describes the results obtained on applying the method of BENEDICT‐BEHRE‐LANGLEY‐EVANS‐LEHNARTZ for the quantitative determination of creatinine to the problem of the methylation of guanidine acetic acid in the animal body. It is found that muscle and liver tissue from the rat are capable of catalysing the methylation of guanidine acetic acid to creatine, the methyl groups being furnished by methionine. The results thus corroborate those of a previou