This report describes the effects of endotoxin treatment on the intrasplenic microcirculation and cellularity in rats. Four and 16 h after a single intravenous injection of endotoxin (2 mg/100g body weight), altered intrasplenic microcirculation was observed. The open circulation was reduced from 97% in the control rats to 79% in the endotoxin treated rats, while the closed circulation increased from 3% in the controls to 21% in the endotoxin treated rats. Such changes in the splenic microcirculation may be partly due to the presence of fibrin and the pooling of polymorphonuclear leukocytes and red blood cells in the red pulp. The most apparent cellular changes seen in the white pulp of endotoxin treated rats 16 h after endotoxin injection are the disappearance of lymphocytes from the periarterial lymphatic sheath and the appearance of many giant macrophages within the white pulp. The giant macrophages contain lymphocytes undergoing various stages of degradation. This suggests that the lymphocytes may be injured by endotoxin treatment and are subsequently phagocytosed by macrophages.