[en] 2-Amino-1,5-dihydro-1-[methyl-²H₃] 4H-imidazol-4-one ([methyl-²H₃] creatinine) was obtained by acidification and heating (120^oC, 120 min) of the [methyl -²H₃] N-amidinosarcosine (creatine) synthesised by methylation of guanidoacetate (GA) with S-adenosyl-[methyl-²H₃] L-methionine ([methyl-²H₃] AdoMet) in the presence of rat liver guanidoacetate methyltransferase (GAA-MT). Isotopic enrichment was 94.7%. (author)