The submicroscopic changes occurring in the liver of rats infected with Plasmodium berghei were studied. The liver tissues were fixed in Palade''s fixative and were embedded in n-butyl metha-crylate. The electron micrographs were taken with a Philips 75 electron microscope. P. berghei was observed in reticulocytes and red blood cells within the liver sinusoid. The fine structure of tropho-zoites illustrated in our study is compatible with the observations of Rudzinska and Trager (1958, 1959). However, we also observed merozoites which had cytoplasmic organelles similar to those of trophozoites. Merozites and trophozites differ in that each merozoite is surrounded by three layers of plasma membrane, whereas trophozoites have double membranes. When merozoites leave the host cell, hemozoin is released from the ruptured cell into the sinusoid and is picked up by reticuloendothelial or Kupffer cells. In our study, early in the infection, a few dense particles were found within lysosomes of Kupffer cells along the inside of the limiting membrane. They were identical in shape and size to hemozoin particles seen in parasites. These hemozoin particles are most probably transferred by endo-cytosis into the lysosomes. In the later stages of infection, the lysosomes become markedly enlarged and are filled with hemozoin particles. Apparently lysosomes cannot break down hemozoin. Since particles accumulate but do not disintegrate, it is conceivable that such particle accumulation may reduce lysosome activity and may interfere with reticuloendothelial function in malarial infection.