Ionic liquids are defined as organic salts that are in a liquid state at room temperature and with melting points below 100°C. Due to their features such as very low vapour pressure, non-flammability, great stability, and especially their recycle potential, ionic liquids are extensively studied as a substitute for traditional organic solvents. To use ionic liquids on the industrial scales, it is necessary to investigate their toxicity and their potential effect on human health and the environment. Therefore, in this study the impact of 1-butyl-3-methylimidazolium tetrafluoroborate on the germination and growth of barley (Hordeum vulgare) is investigated. The results showed that after a 7-day cultivation of barley at different concentrations of ionic liquid, toxic effect on the early growth and development of barley is proven, while the toxic effect grows by increasing the concentration of ionic liquid. Increased activity of antioxidant enzymes superoxide dismutase (SOD), nonspecific peroxidase (POD), catalase (CAT) and ascorbate peroxidase (APOX) and increased content of non-protein thiols was observed, which indicates their participation in the plant defence mechanism against oxidative stress caused by ionic liquids. However, when seedlings were exposed to higher concentrations of ionic liquid, antioxidant system couldn’t effectively remove reactive oxidative particles, leading to lipid peroxidation and damage to the photosynthetic system.