Elevated plasma homocysteine level promotes atherosclerosis in blood vessels due to, among others, generation of reactive oxygen species and reduction of nitric oxide bioavailability. The aim of this study was to investigate whether melatonin administration reduces plasma homocysteine level in rats consuming increased doses of methionine in the diet. The trial lasted for two months. The rats were divided into a few groups - 2 groups consisted of animals fed a standard diet, 2 groups consisted of animals fed a diet rich in methionine for one and two months, a group which had methionine removed from the diet in the second month, a group which had methionine removed from the diet and melatonin administered in the second month, a group still fed a diet rich in methonine in the second month and also given melatonin, and a group of animals on a diet rich in methionine for two months and given melatonin at the same time. Hcy, lipid peroxidation markers (MDA+4HNE) and nitric oxide metabolite (NO(2)(-)/NO(3)(-)) concentrations were determined in the plasma of all the rats. As a result of the tests it was found that plasma Hcy concentration increases in the first month of a methionine-rich diet but then decreases in the second month. MDA+4HNE changes are similar. Melatonin significantly intensifies the effects. The changes of NO(2)(-)/NO(3)(-) concentrations were noticed especially in the groups receiving melatonin. Elimination of methionine from the feed does not change the value of NO(2)(-)/NO(3)(-). NO production increases only after administration of melatonin. On the basis of received results it might be stated that melatonin administration together with a methionine-rich diet significantly decreases Hcy concentration, the level of oxidative stress and increases NO production. It might have some practical implications, especially when the level of endogenous melatonin decreases e.g. in elderly people or people with hyperhomocysteinemia.