This award supplies the funds to upgrade and replace a respiratory gas, mass spectromter which is used by four faculty members at the University of Virginia. The equipment will be used in experiments on small animals (rodents) and in measurements of short-term variability in cardiocascular control. The process by which changes in carbon dioxide levels are sensed by the repiratory control system remains an unsolved problem It is hoped that the projects for which this equipment is intended will help provide a rational explanation for the homeostasis of brain hydrogen ion concentration and thereby to understand how corbon dioxide levels control breathing.