The regeneration of parietal cells was studied in rat gastric mucosa. For this purpose wounds were produced by cauterization of the corpus mucosa in adult male Sprague-Dawley rats. The animals were killed at 30, 60, 90, 130, and 260 days postoperatively, six at each time. Sham-operated rats, killed at 60 and 260 days, served as controls. Stereologic data were obtained by electron microscopy of 10-20 parietal cells in each animal. At 90 days immature parietal cells appeared in the center of the wound. These cells were characterized by large nuclei, small mitochondrial volume density, and few tubulo-vesicles. With further regeneration the parietal cells became ultrastructurally mature, but their total volume in the mucosa remained subnormal. Also the epithelial cell volume density was lower than normal after 260 days of healing.