Blood gas, electrolyte and blood volume changes were followed during 12-hour exposure to aerosols generated from ultrasonic nebulizers. While blood volume and electrolytes remained relatively stable, significant changes in blood gases and A-aDO2 were found. These experiments indicate that atelectasis with increased shunt of blood through perfused but non-ventilated alveoli can result from chronic exposure to high output of ultrasonic aerosols. It was concluded that while ultrasonic nebulizers are an important clinical tool in aerosol therapy, prolonged exposure to such aerosols must be viewed with caution.