The effect of butyrate on cellular morphologic features and morphometric parameters was examined in vitro with clonal glioma cells of the inbred CD Fischer rat. Untreated cells were multipolar with a high nuclear:cytoplasmic ratio, contained few attenuated rough endoplasmic reticula (RER) and cytoskeletal components in the cytoplasm, and were devoid of junctional complexes. Following exposure to butyrate, the glioma cells appeared epithelioid, and each was characterized by a polygonal cell body with a substantially reduced nuclear:cytoplasmic ratio. In contrast to the coarsely clumped nuclear chromatin of untreated glioma cells, the nuclei of the butyrate-exposed cells consisted of finely dispersed chromatin. The cytoplasm of treated cells contained numerous distended RER packed with proteinaceous material and had an elaborate cytoskeletal network. Butyrate also induced the development of junctional devices between apposed surface membranes of adjacent cells. The acquisition of these phenotypic traits more characteristic of normal glial cells makes butyrate a potent, naturally occurring reverse transformation agent.