The development, construction and operation of an open-air fumigation system for exposing young forest trees to controlled concentrations of sulphur dioxide and ozone is described. A computer simulation of gas dispersion was used to design an array of pipework sources which minimized spatial variability in exposure concentrations. Five fumigation plots were constructed using the design and were used to fumigate trees during a 7 year study known as the Liphook Forest Fumigation Project. Rates of gas release were controlled by a small computer to follow predetermined patterns of sulphur dioxide concentration and to maintain an elevation above ambient ozone concentration. Effective control of exposure was demonstrated, and examples of experimentally produced concentration frequency distributions are provided. The advantages and shortcomings of the system are discussed with recommendations for future improvements.