The origin of congenital diaphragmatic hernia (CDH) is considered to lie in a malformation of the nonmuscular primordial diaphragm. It is known that fetal diaphragmatic development requires the structural integrity of its underlying mesenchymal tissue. Developmental mutations that inhibit the formation of normal diaphragmatic mesenchyme have been shown to cause CDH. FRAS1-related extracellular matrix 1 (FREM1) plays a critical role in the development of the fetal diaphragm. It has been demonstrated that a deficiency of FREM1 can lead to CDH both in humans and mice. Furthermore, FREM1-deficient fetuses exhibit a decreased level of mesenchymal cell proliferation in their developing diaphragms. We hypothesized that FREM1 expression is decreased in developing diaphragms of fetal rats with nitrofen-induced CDH.