Parietal cells in the rat gastric mucosa fractured by freeze cracking method under conditions of the control state and tetragastrin stimulation were observed by field emission scanning electron microscopy. The structures thus revealed were compared with those by transmission electron microscopy. 1) In the fractured cytoplasm, intracellular canaliculi lined by numerous microvilli invaginated deeply towards the basal cytoplasm. Typically, microvilli had a club-like structure about 0.6μ in length and 0.2μ in thickness with a bulbous tip and slightly narrowed base. Most of the tops of the microvilli tended to grow up the direction of the canaliculi to the gland lumen, sometimes appearing bent or twisted. Intracellular canaliculi and microvilli differed in shape and size during secretory cycle. 2) Tubulovesicles appeared as many small holes of about 0.05μ in diameter and distributed predominantly in the apical or pericanalicular cytoplasm. Some tubulovesicles directly opened into the secretory canaliculi. Occasionally, tiny microvilli appeared in tubulovesicles. 3) Thirty minutes after tetragastrin stimulation the tubulovesicles increased in size and in number, the small microvilli were more frequently found. Furthermore, a close approximation of the membranes between adjacent tubulovesicles occurred and a 5-layered diaphragm was formed by the fusion of the membranes and changed to a 3-layered diaphragm as the result of the successive elimination of the fused 5-layered diaphragm. In a similar procedure, the membrane of a tubulovesicle apparently fused to that of the secretory canaliculi, and then the depletion of the fused membrane seemed to occur. Tubulovesicles were thus believed to open into the secretory canaliculi and to contribute to the formation of new microvilli. 4) In conclusion, in the stimulated state some amount of the membrane of tubulovesicles was transformed to the membrane of the microvilli in the secretory canaliculi. Consequently, the membrane of tubulovesicles would be preserved as the membrane of the secretory canaliculi, although some of them are believed to disappeare in the process of the depletion of fused membrane. Possibly after cessation acid secretion, the microvilli membrane would be retrieved to the tubulovesicles, but further investigation would be necessary to demonstrate this process.