An antiserum against hemocyanin-conjugated histamine was used to study the cellular stores of histamine in the stomach, especially the oxyntic mucosa, of fetal and early postnatal rats. Tissues were fixed in 4% 1-ethyl-3(3-dimethyl-aminopropyl) carbodiimide (EDC-DI) and standard immunofluorescence technique was used. Histamine was first detected on the 16th embryonic (E16) day when a few histamine-immunoreactive (HA-ir) cells and nerve fibers were observed in the muscular layer of the stomach wall. On day E18, HA-ir cells were visualized for the first time in the oxyntic mucosa of the stomach, and from that day on the number of such cells increased slowly initially and after day E20 more rapidly. At birth many of the HA-ir cells in the oxyntic mucosa possessed processes giving them a paracrine-like appearance typical of enterochromaffin-like cells (ECL cells). Only a very small number of the HA-ir cells represented metachromatically stained mast cells and were located in the submucosa. After birth, the number of HA-ir ECL cells increased steadily, until day 21 when the distribution and number was very similar to that of the adult. The results suggest that histamine-containing neurons and ECL cells appear in the stomach wall before birth, and that there are histamine-containing ECL cells in the mucosa and mast cells in the submucosa of the stomach wall at birth.