A detailed study of the ontogenesis of deer stomach has not been undertaken to date, and our aim was to sequence several histological phenomena that occur during the ontogenesis of one of the gastric compartments, the rumen. Histomorphometric and immunohistochemical analyses were carried out on 50 embryos and fetuses of deer from the initial stages of prenatal life until birth. For the purposes of testing, the animals were divided into five experimental groups: group I, 1.4 -3.6 cm crown-rump length, 30-60 days, 1-25% of gestation; group II, 4.5- in its configuration the stratum basale, the lamina propria and the submucosa. From the pluripotential blastemic tissue at 60 days we witnessed the histodifferentiation of the primitive tunica muscularis, composed of two layers of myoblasts with a defined arrangement. It was also from the pluripotential blastemic tissue, at 97 days, that the lamina propria and the submucosa were differentiated. The serosa showed continuity in growth as well as differentiation, already detected in the undifferentiated outline phase. The tegumentary mucosa of deer rumen was shown without secretory capacity in the initial embryonic phases; neutral mucopolysaccharides appeared from 67 days. The presence of neuroendocrine cells (non-neuronal enolase) in the ruminal wall of deer during development was not detected until 97 days. The glial cells were detected at 142 days for glial fibrillary acidic protein and at 67 days for vimentin. The immunodetection of neuropeptides vasointestinal peptide and neuropeptide Y progressively increased with gestation period, starting from 97 days. In terms of the structure of the rumen of the primitive gastric tube, our observations revealed that the deer is less precocious than small and large domestic ruminants. Thus its secretory capacity, detected by the presence of neutral mucopolysaccharides, and its neuroendocrine nature, determined by the presence of positive non-neuronal enolase cells, were evident in more advanced stages of prenatal development than those detected in the sheep, goat and cow.