Sodium amidotrizoate is characterized by structural similarity with TIBA, and it could be speculated that it functions as a plant growth regulator. The aim of this work was to detect how it influences the growth and metabolism of the smallest vascular plant, Wolffia arrhiza Wimm., that is commonly used in biotechnological treatment of sewage, especially of human and agricultural origin. Sodium amidotrizoate strongly suppressed the growth of W. arrhiza (expressed as a fresh weight) by 21-30%, chlorophyll a by 9-30%, chlorophyll b by 26-30%, total carotenoids by 28-41%, monosaccharides by 6-10% and water-soluble proteins by 10-28% at the range of concentrations of 10^-5-10^-4 M, in comparison to the control. On the other hand, sodium amidotrizoate at lower concentrations (10^-7-10^-6 M) increased the content of analyzed biochemical compounds in W. arrhiza, except of the total pool of carotenoids. After seven days of treatment with 10^-7-10^-6 M sodium amidotrizoate, cultures were analyzed by SDS-PAGE, which showed the presence of some new specific polypeptides. Moreover, 10^-7-10^-6 M sodium amidotrizoate caused a two-fold increase in the activities of ascorbate peroxidase (APX) and NADH peroxidase. 10^-8 M sodium amidotrizoate had no statistically significant effect on the W. arrhiza. Sodium amidotrizoate acts as a plant growth regulator and it could be speculated that it performs activities, similar to TIBA.