Guanidinoacetic acid (GAA), a precursor of creatine, is an essential substrate for muscle energy metabolism, and synthesized by glycine-amidinotransferase (transamidinase) mainly in the kidney. Since the intranephron distribution of transamidinase activity has never been quantified yet, the purpose of this study is to provide evidence about the localization of transamidinase activity using microdissected individual nephron segments. Synthesized GAA was separated by HPLC and detected fluorometrically after reacting with 9,10-phenanthrenequinone. Results obtained were as follows. (1) Transamidinase activity was distributed only in the first (S1) and the second (S2) portion of the proximal tubule, S1 being significantly higher than S2. (2) In S2, arginine and glycine were better substrates for GAA synthesis than canavanine and glycine. These results clearly indicate that GAA is synthesized in definite portions of the proximal tubule, and would be transported to the liver for further creatine production.