1. The argyrophil (enterochromaffin-like) cells in the oxyntic gland area of the rat stomach contain histamine, which can be demonstrated fluorescence microscopically after exposure to gaseous OPT. After administration of L-dopa (or L-5-hydroxytryptophan), these cells produce and temporarily store dopamine (or 5-hydroxytryptamine), demonstrable by its characteristic formaldehyde-induced fluorescence. Ultrastructurally, the enterochromaffin-like cells, which have the appearance of polypeptide hormone-secreting cells, comprise two main cell types, the most predominant one having vesicular type granules (EGL cells), the second most predominant one having smaller, uniformly electron dense granules (A-like cells). 2. Rats were subjected to the following surgical treatments: antrectomy; porta-caval shunting; antrectomy+porta-caval shunting; or sham-operation. Three to eight weeks after surgery the histamine-storing cells (enterochromaffin-like cells) of the oxyntic mucosa were analysed by fluorescence histochemistry, light and (quantitative) electron microscopy, and fluorometric determination of amines. 3. After antrectomy, fluorescence histochemistry and silver staining revealed a reduced number of enterochromaffin-like cells. The histamine content in the oxyntic mucosa was reduced by about 50%. As in unoperated injection of pentagastrin seemed to mobilize histamine. Feeding or injection of insulin failed to do so in antrectomized as opposed to control rats. Ultrastructurally, the cytoplasmic granules of both endocrine-like cell types were less numerous than in the unoperated rats. The reduction in cell number and granularity was particularly conspicuous with regard to the EGL cells. 4. After porta-caval shunting the number of enterochromaffin-like cells increased markedly. Chemical determination revealed a twofold increase in the histamine concentration of the oxyntic mucosa. Feeding or injection of insulin or pentagastrin lowered the histamine concentration. As judged by electron microscopy, the proliferation of endocrine-like cells induced by porta-caval shunting was restricted to the ECL cell type. Besides occurring in greater number, these cells were larger than those in unoperated controls, and their cytoplasm was densely packed with granules that were increased in size. 5. Following antrectomy of the porta-caval shunted rats the number of enterochromaffin-like cells and the oxyntic histamine concentration was reduced. 6. The results support the idea that gastrin exerts trophic as well as excitatory effects on oxyntic endocrine-like cells.