The K⁺-Cl^– cotransporter (KCC) plays a significant role in the cell volume regulation, epithelial transport and ion homeostasis of many cell types (Hebert et al. 2004). So far, four KCC isoforms (KCC1-4) have been cloned. KCC1 and KCC4 are expressed ubiquitously. KCC2 is expressed in neurons in the central nervous system and retina. KCC3 is abundant in muscle, brain, spinal cord, kidney, heart, pancreas and placenta. So far, the expression of KCC isoforms in stomach has not been reported. Here we tested whether KCC isoforms are expressed in rat, mouse, rabbit and human gastric mucosa. Rats and mice were humanely killed by cervical dislocation. Rabbits were humanly killed by the intraperitoneal administration of an overdose of urethane (2 g kg^-1). Isolated rabbit gastric glands were prepared as previously described (Sakai et al. 2003). Human gastric specimens were obtained from surgical resection of Japanese patients with gastric cancer in accordance with the recommendations of the Declaration of Helsinki and with the ethics committee approval. The expression of KCC mRNA and the protein in the samples was examined by Northern blotting and Western blotting, respectively. The distribution of KCC protein in the tissue sections was examined by immunohistochemistry. Using isolated gastric mucosa and the gastric parietal cells of rats and rabbits, we compared the expression level of KCC1, 3, and 4 mRNAs between the mucosa and the parietal cells. Northern blot analysis showed that only KCC3 mRNA is more abundant in the parietal cells compared with the gastric mucosa. In rats, the expression level of KCC3 mRNA in the parietal cells was (4.7 ± 0.2)-fold higher than that in the gastric mucosa (n = 3, P < 0.01). In rabbits, the expression level of KCC3 mRNA in the gastric parietal cells was (2.0 ± 0.2)-fold higher than that in the gastric mucosa (n = 3, P < 0.05). Western blot analysis using a specific anti-KCC3 antibody showed the expression of KCC3 protein (∼160 and ∼190 kDa) in isolated gastric mucosa of rats, mice and humans, and in isolated rabbits gastric glands (n = 3-5). Immunohistochemistry in isolated mucosa of rats, mice and rabbits showed the expression of KCC3 protein in the parietal cells (n = 3-5). In human gastric cancer tissue, decreased expression of the KCC3 protein was observed (n = 3). These results suggest that KCC3 is abundantly expressed in the gastric parietal cells and it may be involved in the mechanism of gastric acid secretion.