OBJECTIVE The aim of this study was to investigate the natural history of congenital diaphragmatic hernia (CDH) in rat embryos with special attention to the pathomorphologic changes of diaphragm, liver, intestines, and lungs at various stages of embryonic development. METHODS Pregnant Sprague-Dawley rats were given, via a gavage tube, 100 mg nitrofen (Wako Chemicals, Neuss, Germany) on day 10.5 of gestation. Fetuses were harvested by laparotomy on day 15.5, 16.5, 18, and 21. Anatomic study of the diaphragm, herniated viscera, and lungs was performed under stereoscope with special attention to the diaphragmatic defect. RESULTS CDH occurred in 44 embryos on right, in 10 on the left, and in 34 bilateral. In the youngest embryos, the small defect was located in the dorsomedial portion of the diaphragm very close to the aortic hiatus. During pregnancy, the defect enlarged progressively to occupy more than half of the affected hemidiaphragm on day 21. In all animals, regardless of their age and side of the defect, the liver was found inside the chest. The intrathoracic mass was formed by ingrown hepatic tissue originating from the dorsal surface of the intraabdominal liver. It appeared as an accessory liver lobe. The amount of intrathoracic liver increased rapidly. From around day 18, the thoracic portion of liver, when examined in sagittal plane, was bent forward and assumed an uncinate shape. The stomach and small bowel loops were displaced into the chest in the oldest examined fetuses. The first sign of reduction of size of the lung was seen in 16, 5-day old embryos, and at further stages of embryonic development the lung growth impairment strictly paralleled the ingrowth of the liver. The natural history of the right and left CDH were very similar. In fetuses with bilateral CDH, asymmetry regarding size of the defect and volume of intrathoracic hepatic mass was noted with larger defects on the left side in more than 60% of animals. CONCLUSIONS Pathogenesis of nitrofen-induced diaphragmatic defect in the rat embryo and CDH in the human fetus seem to differ significantly. Nitrofen induces a complex malformation of the embryonic diaphragm and the liver. CDH in rat embryo bears more resemblance to a model of space-occupying lesion than to human CDH. The induction of localized hepatic proliferation may be a result of disturbance of as yet unknown regulatory interaction between hepatic and mesenchymal diaphragmatic cells.