The organs of monkeys infected with Ebola haemorrhagic fever were examined by light and electron microscopy during the acute stage of the disease. The virus caused focal coagulative necrosis in the liver, spleen, kidney, lung and testis and widespread mild vascular damage. In the brain there was intense congestion, with erythrocyte 'sludging', but no inflammatory reaction. There was significant injury to the microvasculature in all organs. Virus replicated in endothelial cytoplasm causing focal necrosis, separation of tight junctions and detachment from basement membranes. These changes were associated with oedema and haemorrhage, but though contributing to the hypovolaemic shock were not sufficiently extensive to account for the severity of vascular collapse. Renal involvement was also clinically important. Some renal cellular injury was caused by direct virus invasion of glomerular endothelium and tubular epithelium, but much tubular damage was probably due to ischaemia resulting from thrombosis in the peritubular capillaries. The virus also replicated in lymphocytes and monocytes and in interstitial cells of the testis. Since particles were not found in seminiferous epithelium, the degeneration of spermatogonia and spermatocytes was probably secondary to ischaemia.