We studied the effect of restraint stress upon the degree of lipidic peroxidation in the fronto-temporal cerebral cortex, diencephalon, thymus and gastric mucosa. We determined the malon-dialdehyde (MDA) by spectrocolorimetry, the percentage of animals with stress gastric ulcer and the surface of gastric ulcerations. The results demonstrated an important increase of the MDA correlated with frequency and intensity of the gastric ulcer. In the second stage of the experiment, we studied the influence of the ubiquinone and melatonin on the same parameters studied in the first part. In every structures these two substances had antioxidative effect. For ubiquinone, results showed significant variations in the thymus and gastric mucosa, while at the cerebral level the results were doubtful. As antioxidant and antiulcerogenic factor, melatonin was more effective than ubiquinone, because determined significant decrease, or even disappearance of the peroxidative process in the cerebral cortex and the thymus and significantly diminished both peroxidation and ulcerogenesis in the gastric mucosa.