Potassium (K+) balance is achieved by the control of urinary K+ excretion and by the control of K+ absorption from the digestive tract. While it is well established that colonic H/K-ATPase a subunit mRNA is expressed in the kidney, distal colon, and uterus, little is known about the cellular localization and expression levels of this gene in chronic hypokalemia. Accordingly, Northern analysis and in situ hybridization (ISH) were carried out to analyze the expression of mRNA encoding the colonic H/K-ATPase a subunit in normal and potassium-restricted (2 weeks) rats. Northern analysis demonstrated that colonic H/K-ATPase a subunit mRNA was abundantly expressed in normal and potassium-restricted rat uterus. Abundance of colonic H/K-ATPase a subunit mRNA in potassium-restricted rat uterus was increased but, not statistically significant compared to that of controls. By ISH, mRNA for colonic H/K-ATPase a subunit was detected in the endometrial epithelial cells and the uterine glands. Both groups exhibited comparable cellular patterns of labeling, but signal intensity of potassium- restricted rats was higher than normal rats. The endometrial epithelial cells exhibited a mixture of hybridization signal intensity. Most cells had intense hybridization signal for colonic H/K-ATPase a subunit mRNA, some cells had moderate, and a few cells had weak. In summary, colonic H/K-ATPase a subunit mRNA is expressed in the endometrial epithelial cells and the uterine glands. These results suggest that two (or more) H/K-ATPase a subunit isoforms are present in rat uterus and this gene contributes to potassium reclamation during chronic hypokalemia to regulate the pH and/or electrolyte concentration of uterine fluid.