The ret proto-oncogene (proto-ret) encodes a receptor tyrosine kinase that contains a cadherin-related sequence in the extracellular domain, which plays an important role in control of normal cell growth and differentiation. Recent studies have shown that familial Hirschsprung's disease is closely linked to the proto-ret locus. Immunohistologically studies on congenital aganglionosis rats were made using polyclonal antibody generated for the carboxy-terminal sequence of the proto-ret products. Immunoreactivity was investigated in the small and large intestines of mutant and control rats. It was increased in the ganglia of the hypoganglionic segment of the small intestine and was minimal in the normoganglionic small intestinal segment of congenital aganglionosis in rats. The reactivity was also strong in the ganglia of the large intestine of normal rats. Because the proto-ret is preferentially expressed in the neuronal tissue and plays a critical role in differentiation of apparently multipotential neural crest cells to enteric neurons, the results suggest that abnormal development of local vagal neural crest cells, which were influenced by spatially regulating factors, results in the hypoganglionic segment.