Existence and distribution of small guanine nucleotide binding proteins (G proteins) such as smg p25A, smg p21s, and ras p21s were examined in the rat gastric mucosa. By immunoblot analysis using the specific monoclonal antibodies against smg p25A and ras p21 and the anti-smg p21 antiserum, smg p25A and smg p21s were detected in the gastric mucosa, while ras p21s were not detected. In immunocytochemical studies, moderate fluorescence of smg p25A was homogenously seen in the cytoplasmic portions of chief cells and surface epithelial cells. smg p21 immunoreactivity was detected in glandular cells and submucosal muscle layer in the mucosa. On the other hand, the cells constituting gastric glands were unstained with the anti-ras p21 monoclonal antibody, RASK-4. These results suggest that smg p25A and smg p21s exist as in other tissues and might exert their specific actions in the rat gastric mucosa.