Endothelin-1 (ET-1) is the most potent vasoconstrictor peptide secreted by placenta. Its localization in placental tissue of smoking women has been scarcely reported. In this study, ET-1 expression was examined and compared with that of CD-31 (endothelial cell marker) in the placenta of non-smoking and active smoking women using Avidin-Biotin-Complex (ABC) developer kit. ET-1 immunoreactivity was widely distributed in the small and medium sized capillaries and venules in placental villi of both the groups. The distribution of ET-1 in (omit mAb) placental villi was similar to the distribution of CD-31. It was finely granular, dark brown in color and the product of immunoreaction was localized in the cytoplasm of endothelial cells lining the capillaries of placental villi. Specific and high affinity binding sites for ET-1 in placental endothelial cells were noticed in both the groups. No difference was discernible in the distribution pattern of ET-1 in placental villi between smokers and non-smokers group but the ET-1 immunoreactivity in placenta of active smokers was consistently observed to be more intense as compared to non-smokers' placenta. Thus, it is thought that exposure to tobacco during pregnancy may influence the production of ET-1 by endothelial cells in human placenta. Consequently, it is presumed that during pregnancy, the production of ET-1 may be upregulated in the placenta of active smokers which might be responsible for the adverse outcomes of pregnancy due to tobacco exposure.