The cerium (III), lanthanum (III) and neodymium (III) complexes of 5-aminoorotic acid were synthesized and their structures were determined by means of theoretical study, spectral and physicochemical analysis. Significant differences in the spectra of the complexes were observed as compared to the spectra of the ligand. The ligand and the complexes were tested for the antioxidant activity. The results obtained indicate that the tested compounds exerted a considerable antioxidant effect upon the evaluated radicals' formation in the blood plasma of Wistar rat.