Oxygen free radicals attribute to the peroxidation of lipids forming the cellular wall. These processes are manifested by means of phospholipid (PL), triglyceride (TG), and cholesterol (CH) concentration changes. Simultaneously, as a result of arachidonic acid peroxidation, conjugated dienes (CD) and malonyl dialdehyde (MDA) are released. The aim of the study is to investigate these changes in the course of experimental peritonitis (EP). METHODS In 100 rats experimental peritonitis was elicited by means of cecal ligation and puncture (CLP). All animals were split to 2 groups. Group 1 consisted of 40 animals, which survived the 3-day observation period. In group 2 there were 60 rats, which died. Blood was collected in 6-, 12-, 24- 48- 72-hour intervals after surgery or until animal death. Results. Cholesterol concentration changed insignificantly in Group 1 in all time points. Its decrease was statistically significant in Group 2. Triglyceride and PL concentrations insignificantly increased in Group 1. This increased was significant in Group 2. Highest CD concentrations (1.74 IU in Group 1 and 1.89 IU in Group 2) were measured in the early inflammatory phase. In Group 1 MDA concentrations decreased insignificantly, in Group 2--the differences were statistically significantly. The differences of measured parameters were significantly between Group 1 and Group 2. Negative correlation between CD. and TG or CD. and PL was obtained in Group 2 after performing the Pearson test. The correlation between CD. and CH was weak positive. CONCLUSIONS Lipid peroxidation product blood concentrations vary in the course of diffuse peritonitis along with sepsis aggravation. Conjugated dienes' extinction and malonyl dialdehyde concentration are highest in the initial phase of sepsis. Lipid metabolism product concentrations correlate with arachidonic acid peroxidation product levels. The oxygen free radical reaction initiates lipid peroxidation disorders on the molecular level in patients with sepsis.