Summry—The fundic mucosa of resting and acid‐secreting rabbit stomachs were freeze‐fractured and replicated to compare the intramembranous particles on the parietal cell tubulovesicles (rest) and canaliculus (secretion). The particles were counted and their shadow diameters were measured using an image analysis program. The tubulovesicles bore 9 726 ± 400 particles per μm2(mean ± SD), having a mean diameter of 8.4 nm (n = 2571). The canaliculus bore 8 369 ± 430 particles per μm2, having a mean diameter of 7.7 nm (n = 3259). The data were reproducible: three fractures of tubulovesicles and canaliculus gave essentially the same distributions of particle diameters. By contrast, the frequency distributions of tubulovesicle and of canaliculus particle diameters were significantly different (P<0.0005). Neither the opposite curvatures of tubulovesicle and canaliculus microvillus fractures nor subpopulations populations of tubulovesicles with different particle diameters, were the cause of the difference, since there was only one population of tubulovesicles. We therefore postulate that the diameters of intramembranous particles of tubulovesicles and canaliculus are different and suggest, as a working hypothesis, that the difference could be due to a conformational change of the major intramembranous protein, the (H+,