Ibuprofen (IBU) is detected worldwide in water bodies due to the incomplete removal by wastewater treatments. Contrasting results have been reported on the toxicity of IBU on aquatic biomonitor plants such as duckweed, and no data about IBU detection and metabolism in plants has been reported. In this work, the effects of 1 mg L-1 IBU on Lemna gibba L. were monitored in an 8-day laboratory test. In particular, an increase in frond number (+12%) and multiplication rate (+10%) while no variations in photosynthetic pigment content were observed. Moreover, UPLC-HRMS analysis of the presence of IBU and its metabolites in plants and in the growth medium was performed. The results showed that, besides IBU, 11 IBU metabolites were detected in plants. Among the IBU metabolites, hydroxyl- and dihydroxyl-IBU were found, whereas carboxyl-IBU was undetectable. Interestingly, some IBU metabolites were detected in the plant growth solution at the end of the IBU treatment, while no IBU products were found in the IBU solution without plants, suggesting a role for L. gibba in IBU metabolism. The findings of this work represent an important step for a better evaluation of the effects of IBU and its metabolites in duckweed, with notable implications for the eco-toxicological assessment of IBU in the aquatic ecosystem. (C) 2015 Elsevier B.V. All rights reserved.