Biotransformation and metabolic responses of plants to cyanide were investigated using pre-rooted plants of weeping willows (Salix babylonica L.) grown hydroponically in growth chambers and treated with potassium cyanide. Various physiological parameters of the plants were monitored to determine toxicity from exogenous cyanide exposure. Cyanide doses used in this study showed growth-promoting effects on plants, exhibiting higher measured values of transpiration rates, chlorophyll contents and soluble protein contents compared with the non-treated control plants. Superoxide dismutases (SOD), catalase (CAT) and peroxidase (POD) activities in leaves showed a slight change to cyanide application in most treatments. Of all selected parameters, soluble proteins of plants were the most sensitive indicator to cyanide application. Almost all applied cyanide was removed from the hydroponic solution in the presence of plants in all treatment groups. Small amounts of cyanide were detected in the plant tissues. Recovery of cyanide in different compartments of plants varied significantly, root being the dominant sink for cyanide accumulation. Mass balance studies showed that >97% of the applied cyanide was metabolized during transport through weeping willows and the metabolic rates of cyanide by plants were linearly increased with increasing of cyanide applied in the growth media. Results from this study indicated that neither visible toxic symptom nor metabolic lesion was observed for the plants after 192h of exposure, largely due to the well-established detoxification systems in willows. These findings suggest that cyanide has a beneficial role in plants and phytoremediation is a desirable solution of treating environmental sites contaminated with cyanide.