The femoral bone marrow of multimammate rats (n=90), aged 3-8 weeks, experimentally infected with different doses of Trypanosoma congolense was examined by light and electron microscopy. Some animals died from trypanosomosis, but groups of 10 were killed at 4-8, 9-16, 20-24, 30, 40, 50 and 60 days post-infection (dpi). In the acute stage of infection (4-8 dpi) the bone marrow invariably showed a striking increase in erythropoiesis, characterized by an increase in the number of mitotic figures and erythroblastic islands and by a marked decrease in the myeloid:erythroid cell ratio. Later in the infection, erythropoietic activity decreased, while erythrophagocytosis, granulopoiesis, megakaryopoiesis and plasma cell population increased. In chronic infection (16-60 dpi), erythropoietic activity decreased, while intra- and extra-vascular erythrophagocytosis greatly increased. There was also an increase in the bone marrow stroma cells. Excessive erythrophagocytosis by these cells led to the formation of myelin figures and cytoplasmic telephagolysosomes. Degeneration and necrosis of neutrophils lining the adluminal surfaces of the blood sinuses were observed. It is concluded that in the acute stage of the infection, the bone marrow is responsive to the anaemia and that in the chronic stage, dyserythropoiesis and increased erythrophagocytosis by the expanded and activated cells of the mononuclear phagocytic system play an important role in the production of anaemia.