Nitric oxide synthase is demonstrated immunohistochemically in the cytosol, on granules, and on syncytiotrophoblasts membranes. The enzyme is also detected on placental villous stroma cells, and on endothelial cells. The histochemical staining method NADPH-diaphorase stains the syncytiotrophoblasts intensely, and stroma cells more weakly. Membranes of syncytiotrophoblasts immobilized on nitrocellulose paper are also stained by NADPH-diaphorase, and by antisera to nitric oxide synthase. Oxidases of sex steroid synthesis do, however, influence placental trophoblasts and there are discrepancies in the staining pattern of endothelial cells. Caution should therefore be exercised when using NADPH-diaphorase as a staining method for nitric oxide synthase in placenta.