Rats were placed in a decompression chamber and the chamber and the pressure reduced to 265 mm Hg over a period of 1 hr. Other rats were subjected to the same treatment after having been given an injection of frusemide. An ultrastructural study of the lungs of these rats showed that acute decompression of this magnitude can cause swelling and disintegration of type I alveolar epithelial cells. Although capillary endothelial cells showed only minor cytoplasmic damage many capillaries were ruptured with diapedesis of red cells. This is believed to be caused by gross capillary dilatation associated with pulmonary congestion. Pretreatment with frusemide prevented capillary rupture and reduced the degree of epithelial degeneration. However, frusemide induced swelling of capillary endothelial cells which was unassociated with acute decompression.