A H2-He-O2 mixture with 54 to 56% hydrogen was studied with 6 subjects (professional divers) during 2 dives to 450 m. The 38-h compression was the same as that used with other types of breathing mixtures (He-O2 and He-N2-O2). The results obtained during compression and during the stay at 450 m in H2-He-O2 show that the EEG changes (increase of theta activities in the anterior regions of the skull, decrease of alpha activities) are similar to those found with other respiratory mixtures. On the other hand, the other symptoms of high pressure neurologic syndrome (HPNS) were clearly improved for the same depths. Thus, neurologic symptoms (tremor, dysmetria, myoclonia, drowsiness) are nonexistent, and the performances during psychometric tests remain similar to those of the surface. Hydrogen, with its narcotic potency, suppresses some symptoms of HPNS and seems to open new perspectives for deep diving.