The results of the study of the fluorescence spectra of suspensions of synaptosomes, which have been exposed to a chlororganic pesticide, thiamethoxam, at a concentration of 50 MPC during different time periods, at the excitation/emission wavelengths of 290 ± 5/310-600, 340 ± 5/360-700, and 420 ± 5/450-800 nm are given for the first time. It has been demonstrated that the development of intoxication results in weakening of the fluorescence intensity of tryptophan, NAD(P)·H, derivatives of vitamin B, and vitamin А and in an increase in the fluorescence of pyridoxic acid, lipofuscin, and flavin and porphyrin complexes. The results of the spectral studies indicate that the toxic effect of the chlororganic pesticide for the functioning of living systems is based on free radical toxicity. [ABSTRACT FROM AUTHOR]