Uniform delivery of dry material for stable concentrations of aerosols in inhalation exposure chambers is essential in inhalation experiments. The paper characterizes an AccuRate dry material feeder with modifications, for different helix sizes, actuation rates, nozzle types and dye mixtures. The basic design of the AccuRate dry material feeder used to deliver precise quantities of dye mixtures is inadequate for aerosol exposure research. To improve delivery, a push-pull type solenoid was mounted to the feeder and connected to a microprocessor controlled timer for actuation. Without the actuator, the coefficient of variability increased with smaller size helixes and reached 35, 34 and 121% for 1/2, 3/8, and 1/4 inch helixes, respectively. With the actuator, the coefficient of variability was reduced to 2.5, 4.8 and 8.8% for the same helix sizes and red dye mixture. The optimum actuation rate was determined to be 90 cycles/min. The use of the stainless steel nozzle improved feed rates by presumably limiting any additional amount of electrostatic charge on dye particles. The modification, addition of actuator with rate of 90 cycles/min to the AccuRate feeder greatly decreased the amount of variability in feed rates for all helixes and dye mixtures, while increasing the dye flow rates. The modification is recommended when dry material characteristics are such that accuracy and precision of feed rates cannot be optimized. (Copyright (c) 1991 by Hemisphere Publishing Corporation.)