A procedure for the quantitative determination of hydrogen cyanide (HCN) released by plants has been developed based on the UV–vis spectrum of the sodium picrate–cyanide complex. Fresh plant tissue mixed with toluene was placed in a gas flow system designed to carry the evolved HCN through 2,4-dinitrophenylhydrazine solution in acidic water:ethanol to trap interfering volatile carbonyl derivatives, and then into an alkaline solution of sodium picrate. After 18 h of gas flow at a rate of 6 mL/min, the absorbance of the solution was measured at 500 nm and the concentration of HCN was determined by calibration in the range 10−3–10−5 M. The molar absorptivity coefficient (1.385 L/cm M) yielded a detection limit of 2.6 × 10−6 mol/L and a 92.6 ± 2.6% recovery yield of HCN. The method was applied to determine the cyanide release capacity of Passiflora capsularis (up to 3.34 mg of HCN/g fresh plant tissue), and of croziers of Pteridium aquilinum var arachnoideum (10.4–61.3 mg of prunasin/g fresh plant tissue), and its rate of HCN production (K = 2.20 ± 0.01 × 10−4/s). Cymbopogon citratum, known to release large quantities of volatile, potentially interfering, monoterpene ketones and aldehydes, gave a negative reaction. Copyright © 2000 John Wiley & Sons, Ltd.