In a child with severe muscular hypotonia and extrapyramidal movement disorder, elevation of guanidinoacetate and deficiency of creatine in brain suggested a defect in biosynthesis of creatine at the level of guanidinoacetate methyltransferase (GAMT) (Stöckler, S. et al. (1994) Pediatr. Res. 36, 409-413). As a first step in the molecular analysis of GAMT deficiency, the cDNA for human GAMT was cloned using rat GAMT cDNA as a probe. For the coding region there is a 82.5% homology of the nucleotide sequence and 86.9% homology of the deduced amino acid sequence between human and rat.