Objective: to study the activity of proteolytic processes and oxidative modification of proteins in different environments of the mother-placenta-fetus system in preeclampsia. Subjects and methods. Examinations were made in 62 women, including 34 with preeclampsia-complicated pregnancy, who were a study group, and 28 women with physiological pregnancy, who were a control group. Spectrophotometric studies were conducted in the investigation. Results. In preeclampsia, there was a significantly enhanced activity of peptide hydrolases acting at different pH values. The highest magnitude of changes in the activity of these enzymes was established in the placenta. Abnormalities were less marked in serum and amniotic fluid. There were differences in the rate of proteolytic processes in some placental subcellular fractions. Placental protein destructive changes caused by peptide hydrolases occurred during their oxidative modification. These processes showed a direct correlation suggesting the relationship of these reactions. Conclusion. The found impairments in the pattern and rate of peptide hydrolase destruction of proteins in different biological environments of pregnant women with preeclampsia unveil the specific features of its pathogenesis at the cellular level. The authors declare no conflicts of interest.