Nanoparticles are substances with all three dimensions less than 100 nm. Due to the larger active surface area per mass, nanoparticles have proven to be biologically active than their "larger duplicates" (particles of micrometer dimensions) of the same chemical composition. Increased use of nanoparticles results in their release into the environment. This thesis examines the effect of silver nanoparticles in the concentration range of 0.1,0.5,1,2 and 5 mg L-1) on duckweed (Lemna minor L.), which is often used as a test plant. A static test design of the standardized Lemna toxicity test (a growth inhibition test) based on growth rates in the duration of seven days was applied. In addition to growth, Ag content of media and plant material, chlorophylls, carotenoids and certain indicators of oxidative stress in duckweed have been determined. Silver nanoparticles significantly reduced growth rate and content of photosynthetic pigments. On the other hand, metal nanoparticles caused oxidative damage to lipids as well as the modulation of antioxidant enzyme activities (superoxide dismutase, catalase and ascorbate peroxidase). The obtained results confirm the toxicity of silver nanoparticles to duckweed, which is probably due to increased accumulation of silver nanoparticles in plant tissue. Furthermore, the results indicate that oxidative stress is involved in the toxicity mechanism of silver.