2,5-Dihydroxybenzylidene aminoguanidine (BAG) is a structural analogue of the antidiabetic compound aminoguanidine, and is an example of a substance protecting diabetic rats from lipoprotein oxidation arising in oxidative stress conditions characteristic of diabetes mellitus. We found that administration of BAG to diabetic rats decreases their susceptibility to lipoprotein oxidation, decreases formation of conjugated dienes and 4-hydroxy-2-nonenal, and increases antioxidant potential of plasma. On the other hand, our results show that BAG has a negative influence on lipoprotein oxidation in control rats. Increased formation of 4-hydroxy-2-nonenal and conjugated dienes and a decrease in plasma antioxidant potential was observed when BAG was administered to control rats. It is therefore necessary to search for other structural modifications of this substance that would combine higher antidiabetic activity with less toxicity in healthy individuals.