RACTK1 cDNA encodes a pH-sensitive K+ channel from the renal cortical collecting tubule cells, which is only localized in the kidney by Northern blots (Suzuki, M., K. Takahashi, M. Ikeda, H. Hayakawa, A. Ogawa, Y. Kawaguchi, and O. Sakai. Nature Lond. 367: 642-645, 1994). We further investigated the localization of the channel by immunohistochemistry and reverse transcription-polymerase chain reaction (RT-PCR). Immunohistochemical analysis showed that the signal was detected in various epithelia, including the luminal membrane of the cortical collecting tubules, artery of the kidney, and pancreatic ducts. Interestingly, the signal was also detected in the coronary vascular smooth muscle and cerebral artery. These tissue distributions were confirmed by localization of mRNA determined by RT-PCR. These findings suggest that RACTK1 encoding pH-sensitive K+ channels is widely distributed in epithelia as well as in vascular smooth muscle cells.