The amiloride-sensitive epithelial sodium channel (ENaC) plays a key role in sodium reabsorption in the collecting ducts. We examined ENaC mRNA distribution along the nephron and acute effects of vasopressin and hyperosmolality on ENaC mRNA expression. ENaCalpha, beta, and gamma mRNA expressions were observed in cortical, outer medullary and initial inner medullary collecting ducts (CCD, OMCD and ilMCD, respectively). ENaCalpha mRNA expression was also observed in medullary and cortical thick ascending limbs (MAL and CAL, respectively), while ENaCbeta and gamma mRNA expressions were not observed. Furthermore, ENaCalpha mRNA expression in MAL but not in collecting ducts was stimulated by acute exposure to arginine vasopressin (AVP), 8-(4-chlorophenylthio) (CPT)-cAMP and hyperosmolality. However, the physiological significance of these effects is not known, since ENaC protein is reported to be absent in MAL. These data suggest that ENaCalpha mRNA expression in MAL but not in collecting ducts is acutely regulated by AVP and hyperosmolality. The absence of stimulation of ENaCalpha mRNA expression in collecting ducts suggests the physiological significance of ENaCbeta and gamma mRNA for acute regulation by vasopressin. Determining the physiological significance of the acute effect of vasopressin in MAL will require further investigations.