1. In man and in various animal species, absorption of NaCl from bile by the gallbladder mucosa is associated with luminal proton secretion. A similar absorption of NaCl in small intestine, colon and renal tubule is related, at least in part, to the presence of the NHE3 isoform of the Na+/H+ exchanger. This work was designed to find out whether NHE3 is also present in human gallbladder. 2. At surgery, 100-200 mg of the gallbladder wall was obtained from patients treated by cholecystectomy for gallstones. After isolation of the mucosa, total RNA was extracted and submitted to reverse transcription-polymerase chain reaction with two primers: 5'-AAGCCICTGGTGCAGTGGCTGAAGG-3' and 5'-GGAGTCCTTIAAGTCGGCIAAGCTGGGC-3', designed to amplify a sequence of 645 bp of rabbit NHE3 mRNA (642 bp in man). RNA from human liver and from rabbit heart, neither of which contain NHE3, and human ileal RNA, which does contain NHE3, were used as controls. 3. RNA extracted from the mucosal moiety of the gallbladder wall gave an amplification product of about 645 nucleotides. Controls gave the expected negative or positive results. Sequencing of the amplified RNA showed it was almost identical to previously determined sequences of NHE3 in other human tissues. 4. It is concluded that the mucosa of human gallbladder contains the mRNA of NHE3 isoform. This isoform could therefore play a role in sodium absorption from bile.