The work is to observe ultrastructure of pituitary gland of rats exposed to electromagnetic pulse (EMP, 200 kV m,-1, 200 times) with transmission electron microscope and to explore the effects of EMP on the endocrine system. At 6h and 12h after EMP exposure, the intercellular spaces were dilated, swellen mitochondria with cristae loss, dilatation of Golgi complex and diffused lysosomes were found in endocrine cells of pituitary gland, especially in prolactin cells and growth hormone cells. There were macrophages infiltrating in intercellular spaces at 24h after EMP exposure, and more mitochondrial vacuolization in prolactin cells, many myelin figures in growth hormone cells and distinct dilatation of endoplasmic reticulum in gonadotropic cells were found. These changes were gradually aggravated. Numerous secondary lysosomes, the gathering of heterochromatins under the nuclear membranes, and cell degeneration and necrosis could be observed at 48h. These results suggest that EMP may damage endocrine cells in pituitary gland of rats, mitochondria and endoplasmic reticulum are the sensitive organelles, and prolactin cells and growth hormone cells are perhaps the sensitive cells to EMP.