Electron microscopy of 18 human pituitary adenomas revealed vascular abnormalities consisting of thickening of endothelial lining, endothelial swelling, endothelial blebbing, accumulation of microfilaments in the cytoplasm of the endothelial cells, partial or total loss of fenestration, and, rarely, thickening of basement membrane. Disturbance of pituitary microcirculation, possibly due to alteration of intrahypophysial pressure, can result in hypoxia. This might play an important role in the genesis of changes of vasculature. Lesions of the pituitary vessels occur not infrequently. If severe, they may interfere with exchanges of O<sub>2</sub> and metabolic products and may alter permeability across the vascular walls. Their further progress may cause irreversible damage to the tumor cells, leading ultimately to hemorrhage and infarction of the tumor.