The object of this research is to determine how information carried on the pulmonary CO sub 2(t) and PO sub 2(t) signals effect period and depth of breathing. And to perfect ways of studying biological control systems with minimal disturbance of the normal balance of the feedback control loops. A unique feature of this research is a unidirectionally ventilated bird in which air is forced into the trachea, passes through the lung and air sacs and then out through surgically placed exit tubes to the outside. Air flow is kept high and constant and the CO sub 2 and O sub 2 concentrations independently forced and respiratory period and tidal volume monitored with a whole body plethysmograph. Since the birds' respiratory movements no longer influence significantly the concentrations of pulmonary CO sub 2 and O sub 2 it is an open-loop system. This system may be made into an externally closed-loop one by using an analog computer to set the CO sub 2 (or O sub 2) level. Observing the input (CO sub 2, O sub 2)-output (respiratory period, tidal volume) relationship provides constraints for modeling this biological control system.