The route of excretion of lipopolysaccharide (LPS) and its possible degradation in vivo was studied in rats using biosynthetically radiolabelled LPS from Salmonella abortus equi, carrying 3H activity exclusively in fatty acids and 14C activity in fatty acids and sugars. Following intravenous injection of the above LPS in AS2 rats with or without anaesthesia, excretion of radioactivity occurred mainly in the faeces and to smaller extent in urine. The rate of excretion was slow, a large part of the radioactivity being still present in the liver after 14 days. In faeces the percent recovery of 3H (18%) was lower than that of 14C (32%) suggesting loss of tritium activity and thereby of fatty acids from the excreted LPS. A similar loss of tritium was also found in the material remaining in the liver and spleen 14 days after LPS administration. In urine the material recovered during 14 days (about 7% of injected) was different from the original LPS, 70% of 14C activity being dialysable and practically all 3H activity being volatile. Similar results were also obtained following administration of the LPS intraperitoneally under anaesthesia. However, when the LPS was administered intraperitoneally without anaesthesia, in the majority of the animals, 90% of 14C and 54% of 3H was excreted in faeces within 3 days, suggesting that both route of administration and use of anaesthesia during injection influence the subsequent rate of excretion of LPS.