Hyperaccumulators are ideal plant species used for phytoremediation of soils contaminated by heavy metals. A full understanding of metal tolerance mechanisms of hyperaccumulators will facilitate enhancing their phytoremediation efficiency. However, how Cd affects N metabolism and which role plays the response of N metabolism to Cd toxicity in the tolerance of hyperaccumulators are still unknown. To clarify these questions, this study investigated the effects of various soil Cd levels on the concentrations of N forms and the activity of key enzymes involved in N metabolism in leaves of the Cd hyperaccumulator, Solanum nigrum L. The results showed that its growth and all N metabolism indicators were normal at low Cd exposure (<= 12 mg kg(-1)). At 24 mg Cd kg(-1) soil, nitrate assimilation indicators (nitrate concentration and activity of nitrate reductase) were reduced significantly, whereas most ammonia assimilation indicators (ammonium concentration and activity of glutamine synthetase) remained normal. However, when exposed to a higher Cd level (48 mg kg(-1)), growth and most N metabolism indicators were reduced significantly. Therefore, N metabolism in leaves of S. nigrum could be tolerant of Cd toxicity to a certain extent (soil Cd concentration <= 12 mg kg(-1)), and this might be involved in the Cd-tolerance of this Cd-hyperaccumulator. (C) 2007 Elsevier B.V. All rights reserved.