The design and use of a variable-dosage spraying machine for the experimental evaluation of herbicides has been reported by Pfeiffer et al. (1). This equipment sprays at constant volume while simultaneously diluting the spray mixture exponentially. The machine is composed of a series arrangement of diluent tank, concentrate tank, pump, and nozzles. The concentrate tank is equipped with a rapid mechanical agitator. The pump propels the spray liquid from the concentrate tank through the nozzles in the conventional manner. The liquid withdrawn from the concentrate tank is replaced by an equal volume of water or other diluent from the diluent tank. Thus the concentration of active ingredient in the spray decreases logarithmically with the volume of spray delivered. Since spray volume is constant with time, by moving the sprayer at constant speed, the rate of application of the active ingredient becomes a logarithmic function of distance along the plot. The machine is mounted on a four-wheel-drive light truck and draws power for the pump and agitator from the power take-off.