Guanidinoacetate N-methyltransferase (GAMT) catalyzes the last step of creatine biosynthesis and the resultant creatine plays an important role in cellular energy metabolism. GAMT is mainly found in liver, kidney as well as testis and epididymis. We have localized the site of creatine biosynthesis in mouse epididymis by immunoperoxidase staining of GAMT using anti-GAMT antibody. Gamt is extensively expressed in the microvilli of epididymal epithelial cells and also expressed weakly in the cyto plasm of the cells. The staining of GAMT was most prominent in the microvilli of proximal caput epididymis and the intensity was progressively diminished as the epididymal tubule proceeds toward caudal part. The result suggests that GAMT or Cr might be involved in sperm function and/or maturation process in epididymis.