Expression of proteins encoded by the ras proto-oncogenes was examined immunohistochemically in formalin-fixed, paraffin-embedded tissues of the normal rat using anti-ras p21 antibodies generated against synthetic peptides. Cell type specific expressions of ras gene products were detected in distal tubules of kidney, megakaryocytes in spleen, neural cells in cerebrum, Purkinje cells in cerebellum, cells lining the pulmonary alveoli and cells in the epithelium of intestinal villi. Region specific expressions of the ras proteins were observed in spleen and thymus, where the ras proteins were detected in splenic nodules including germinal centers and thymic medulla, respectively. These findings suggest that the c-ras gene products in normal rat organs are expressed in specific cell-types within a tissue and may be associated with degree of cellular differentiation.