Many reports have indicated that estrogens increase uterine guanosine 3',5'-cyclic monophosphate (cGMP) levels via increasing the activity of guanylate cyclase. In the present study, guanylate cyclase activity was studied cytochemically in the oviduct of immature rats 24 hours after one or two doses (20 micrograms/kg, subcutaneously) of diethylstilbestrol (DES), one dose per day. The reaction product indicating guanylate cyclase was localized in the plasma membrane of epithelial cells, endothelial cells, and smooth muscle cells of all DES-treated animals, but was absent from those of the vehicle control immature rats. The Golgi saccules of secretory cells and the periciliary membrane of ciliated cells were stained for the enzyme 24 hours after the first and second dose of DES, but the activity seemed diminished 24 hours after the second dose. Likewise, reaction product indicating guanylate cyclase was more prominent in the plasma membrane of epithelial cells of animals treated with one dose of DES than those animals treated with two doses of DES. However, in the endothelial and smooth muscle cells, guanylate cyclase activity increased after two doses of the estrogen. Further, pinocytotic vesicles or caveolae in these cells were also strongly stained for the enzyme after one and two doses of DES. These findings confirm the suggestion that guanylate cyclase plays a significant role in the growth, differentiation, and function (secretion and transport) of the oviduct.