Principles of operation and technical details of an aerosol exposure system for conducting chronic inhalation studies with up to 102 individually caged rodents are described. It provides for the simultaneous whole-body exposure of the animals in a single tier, is easy to operate, and requires relatively little maintenance. The system has been tested in several exploratory studies with aerosols of CoO, NiO, and chrysotile asbestos. It is now being used for long-term inhalation studies with these aerosols. Mean concentrations of respirable aerosols were, for example, maintained during one 5-month period within standard deviations of approximately 20%. Aerosol concentration measurements and remarkably little variation in the quantity of dust deposited in the lungs of exposed hamsters showed that the aerosols were evenly distributed across the diameter of the chambers.