Electromyographic (EMG) recordings of the 6th to 7th intercostal space (thoracic EMG) and abdominal muscles, ventilatory pattern, and the work of breathing were studied in 4 human subjects exposed for 12 days to 46 ATA of helium-oxygen (density = 8.7 g.liter-1) then of nitrogen-helium-oxygen gas mixture (ternary mixture) (density = 11.1 g.liter-1). We found that the respiratory muscle work necessary for eupneic ventilation was multiplied by 4 at 46 ATA. During quiet breathing as well as during forced inspiratory maneuvers, the power spectrum of thoracic EMG shifted to the left in three individuals during the sojourn at maximal pressure, whichever gas mixture was inhaled. This was corroborated by the decreased ratio of EMG power in a high to that in a low band of frequencies. These alterations disappeared at the end of the decompression period, suggesting the existence of inspiratory muscle fatigue at high pressure. Hyperbaric tremor was recorded on the thoracic EMG and was maximal with He-O2 inhalation. It disappeared at the end of the period at 46 ATA (He-N2-O2).