Chemicals taken up into plants may be accumulated so leading to toxic effects. Uptake and phytotoxicity of free cyanide was determined with the willow-tree transpiration test. Willow sets were grown in sand and irrigated with varying levels of cyanide (CN). Toxicity was determined by measuring transpiration. At CN concentrations below 10 mg/L, no toxic effects were observed. At 20 mg/L, transpiration was reduced to approximately 50% after 96 h. With 30, 40 and 50 mg/L, the transpiration decreased with a similar rate to < 20% of the initial transpiration within 96 h. Accumulation of cyanide in plant tissue was observed at 40 and 50 mg/L. The kinetics of metabolism of cyanide by roots, stems and leaves of willows was determined by the closed-bottle metabolism test. The Michaelis−Menten parameters vmax and KM (maximal metabolic velocity and half-saturation constant, respectively) were determined by nonlinear regression. Estimates of uptake and metabolism were balanced using a nonlinear mathematical model. The model predicted that at low doses (<10 mg/L), the cyanide would be rapidly metabolized. At higher doses, uptake would be faster than metabolism and consequently cyanide would accumulate in the plant tissue. This relation between external dose and internal accumulation is nonlinear and explains the toxic effects observed.