It was studied the impact of a water-alcohol extract (70%) from the marine green algae Ulva lactuca L., enriched with a polyphenol fraction, and a herbal preparation – extract of Eleutherococcus senticosus (Rupr. & Maxim.) Maxim. (Araliaceae) on the indicators of antioxidant protection of the liver and blood plasma of rats under acute stress (vertical fixation by the dorsal neck fold). The total polyphenols and flavonoids content in the extract of U. lactuca made 16.2±1.8 µg GAE/g and 9.10±1.87 g QE/g, respectively, which was 2 times higher than its amount in the Eleutherococcus extract – 7.6±0.25 µg GAE/g and 4.8±0.3 µg QE/g respectively. The antiradical activity against ABTS+ and alkyl-peroxyl radicals of the U. lactuca extract was also almost 2 times higher in compere with Eleutherococcus. The pharmacological effect of the phenolic complex of U. lactuca in Wistar rats under stress was manifested in the stabilization of the antioxidant defense system. It was noted a decrease of the malondialdehyde content, an increase of the antiradical activity level, reduced glutathione content and the activity of enzymes of glutathione cycle in the liver and blood plasma of rats. In terms of efficiency, the extract of U. lactuca was not inferior to the reference preparation Eleutherococcus, and in terms of the ability to restore the activity of antioxidant enzymes and indicators of the glutathione system, it was superior to that. The pharmacological effect of the phenolic complex of U. lactuca, to our opinion, is provided by the action of its constituent flavonoids, phenolic terpenoids, phenolic acids (hydroxybenzoic, hydroxycinnamic, coumaric, synapic), etc., which have pronounced antioxidant properties, which prevents the development of lipid peroxidation and oxidative stress. Thus, the marine green algae U. lactuca is a promising raw material for the development of preparations that can increase the potential of the endogenous antioxidant defense system of the body at stress-induced disorders.