1. Respiratory responses to sustained static forearm and leg contractions performed separately and simultaneously were investigated in human volunteers.
2. Oesophageal pressure measurements indicated no tendency to breath hold or fix the thorax during fatiguing static contractions.
3. Marked hyperventilation occurred invariably with the static efforts, resulting in striking increases in ventilatory equivalent and reduction of end-tidal CO2. Simultaneous contraction of arm and leg muscle groups did not result in additive respiratory responses.
4. Contractions performed during circulatory occlusion or cool water immersion of the involved limb produced results which led to the conclusion that the hyperventilation is not due to stimulation of pain fibres, nor to stimulation of chemoreceptors locally or centrally by metabolites released from the active muscle. Some additional possible mechanisms responsible for the hyperventilation are discussed.