The role of serotonergic system in the phenomenon of preconditioning and protection of fish tissues from impact of neonicotinoid insecticide actara has been studied. Sub-lethal concentration of insecticide actara was calculated for the specimens of sazan (Cyprinus carpio Linne) with the application of the Finney method. The resistance to the lethal concentrations (400 mg/L, 5 days) of insecticide actara in the fish was created using the preconditioning procedure (100 mg/L, 1 day). The level of serotonin-modulating anticonsolidation protein (SMAP; Mekhtiev, 2000), being in linear relationship with serotonin, was measured in the fish brain and liver using the indirect solid-phase ELISA-test. The effects of i.m. administration of SMAP on survival of sazan specimens under impact of the lethal concentrations of insecticide were studied. Using the technique by Asakawa and Matsushita (1980), lipid peroxidation levels were determined based on the levels of malone dialdehyde and hydroperoxides in the brain, liver and gills of the sazan specimens subjected to the impact of sub-lethal concentrations of insecticide and i.m. administered with SMAP. Sublethal concentration of insecticide actara for sazan specimens was equal to 360 mg/L. It was shown that artificial upregulation of serotonergic system as a result of pre-conditioning towards insecticide actara or i.m. administration of SMAP to the sazan specimens, subjected later to the impact of lethal concentrations of insecticide actara, promotes their survival and normalization of lipid peroxidation in their tissues. The conclusion concerning anti-apoptotic activity of SMAP towards the impact of the lethal concentrations of neonicotinoid insecticide actara in the sazan specimens has been made.