Nurelle D 550 EC preparation (Dow Elanco, USA) was applied in the study.The contents of biologically active substances in 1l of the preparation was: 500 g chlorpyrifos and 50g of cypermethrin. The experimental studies were conducted on 4 groups of rats (2 control - 10 animalsin each group, and 2 experimental - 40 animals in each group). The experimental groups were dermallyadministered a mixture of 27.8 mg/cm(2) of chlorpyrifos and 2.7 mg/cm(2) of cypermethrin for a periodof 1 week and 4 weeks (4 h daily). The control groups did not participate in the experiment. Blood wastaken from the heart after 1 day, 1 week, 2 and 3 weeks after the administration of the preparation inorder to determine cholinesterase activity in serum, and the brain was taken to evaluate brain cholinesteraseactivity. The brain for histologic studies was taken from rats 3 weeks after the experiment. The cholinesteraselevels in the serum and brain initially decrease and then return to normal at 2 and 3 weeks post-exposure,respectively. Slight histopathological changes in various areas of the brain as well as increased densityof the cytoplasm in neurocytes in both experimental groups were observed 3 weeks post-exposure.