A selective, sensitive, rapid and simple-handling analytical method for the determination of cyanide at low detection limits in surface and underground water, soil and industrial waste samples was developed. The method is based on a reaction, proposed by Guilbault and Kramer, where free cyanide reacts with p-nitrobenzaldehyde to form an intermediate cyanohydrin, which reacts with o-dinitrobenzene to give a highly colored purple compound. The original procedure was modified for application in a small device containing a gas-permeable membrane. The cyanide is converted in the volatile hydrogen cyanide, which permeates through a PTFE membrane, reaching colorimetric reagents. In order to obtain semi-quantitative results, printed color scales were built. The method allows rapid, accurate, selective, low-cost and simple-handling determinations of free cyanide, even in complex samples. About 150 real samples were analyzed. Less than 10 ng of free cyanide per ml (10 microg l(-1)) can be easily detected. For more concentrated solutions, the results had been compared to those obtained using differential pulse polarography. The standard addition method was used for more diluted solutions.