Abstract: Objective To study the effect and significance of high power pulsed mic row ave radiation on pituita ry function and ultramic rostructure of the hippocam pus neurons in rats.Methods Fifty-four male Wistarrats were sacrificed at 6 h,24 h and 48 h after exposure to 100 mW/cm²,200 mW/cm² microw averadiation.Immunoh is tochemistry and electron microscope were used to study POM C derived peptides in both pituitary and serum,and observe the structural in jury characte ristics of the hippocam pus.Results The contents of β-Ep(P<0.01)in pituitary decreased significantly,β-Ep(P<0.05)and A C TH(P<0.01)in serum increased markedly after radiation of 6 h.Neurons of hippocam pus were with irregular nucleus and milds welling of mitochodria in rats exposed to 100 mW/cm² microwave radiation of 48 h.Mildirregular nucleus,unsmoothed nuclear membrane,cavitation of mitochond ria and irregular nucleus,chromatin condensation,loss of mitochond rialcrista were obse rved after exposure to 200 mW/cm² microwave radiation of 6 h and 48 h,respectively.Conclusion High power pulsed microwave radiation can damage the neurons of hippocampus in rats at ultram icrostructural levels and induce hypothalam us and pituitarynerve in ternal secretion disorder.Heats tress may affect hypothalam us-pituitary-adrenaline axis fuction and increase the tole rance to heat stress in rats.