Betaine is a major compatible osmolyte in the renal medulla. It is taken up into cells via the betaine gamma-amino-n-butyric acid transporter (BGT-1). We investigated the localization of BGT-1 mRNA and its acute regulation by NaCl and furosemide administration. In situ hybridization revealed that BGT-1 mRNA is predominantly present in the outer medulla and papilla. Less intense signals were seen in the inner medulla and no signals were found in the cortex. Microscopic examination suggested that intense signals were present in the medullary thick ascending limbs of Henle's loop (MTAL) and the inner medullary collecting ducts (IMCD). A reverse transcription and polymerase chain reaction assay of individual microdissected segments along the nephron confirmed its localization. Intraperitoneal administration of NaCl rapidly increased the signal in the MTAL, and furosemide prevented the increase in BGT-1 mRNA by NaCl loading. In contrast, BGT-1 mRNA in the IMCD is less sensitive to these kinds of acute regulation. These results suggest that BGT-1 expression in the MTAL is rapidly regulated in response to the magnitude of NaCl absorption, as suggested for the expression of Na+/myo-inositol cotransporter.