This study was undertaken to determine if the observed increase in ventilation during passive limb movement was a reflex hyperventilation or a response to an increased metabolic need for oxygen. Experiments on human volunteers were designed to test the hypothesis that the rapid increase of ventilation at the onset of exercise was due to stimulation of the joints. Results of these studies showed significant increases in ventilation, oxygen consumption, carbon dioxide production, ventilation/oxygen consumption ratio, and heart rate compared to rest and recovery values. The data lead to the conclusion that the rapid increase of ventilation at the onset of exercise is a true hyperventilation and that stimulation of the joints can be a significant contributor to increased pulmonary ventilation.