We have studied the effects of two endotoxins, S enteritidis and E coli on liver, pancreas, intestine, lung and kidney of 10 day old Sprague Dawley rat at light and electron microscope levels. One group of experimental animals (N = 31) received a single intraperitoneal injection of 0.1 mg/kg of S enteritidis endotoxin. The second experimental group (N = 34) received multiple intraperitoneal injections of 5 to 10 mg/kg of E coli endotoxin at 15 minute intervals over a period of 75 minutes. Within four hours of injections, histopathologic evaluation of all the tissues revealed interstitial edema, widening of blood capillary lumina, accumulation of polymorphonuclear leukocytes and platelets in capillary lumina and marked swelling, vacuolization and focal desquamation of endothelial cells in microvasculature. Hepatocytes showed loss of glycogen, vacuolization and degeneration in the centro-portal region. Islet cells of pancreas also revealed swelling and vacuolization. In the small intestine, hemorrhagic pools of blood were frequently seen in lamina propria and the apical portion of villi showed degeneration and breakdown. Lungs showed focal hemorrhage, collapse of alveolar architecture and swelling of endothelium in larger artereoles. Based on these studies, we suggest that endothelial cells of microvasculature in various tissues of 10 day old rat are extremely sensitive to endotoxins, irrespective of source, and cells derived from reticuloendothelial system may play an important role in the endotoxicosis.