Using a breathing medium of 40 kPa oxygen, remainder helium, 18 volunteer subjects participated in a series of 15 exposures to pressures equivalent to depths of 180-540 m s.w. The time of exposure at these pressures was mostly 2 days, except for the 540 m s.w. exposure, when 6 days were spent at full pressure. Compression procedures, based upon placing `stages' at 60 m s.w. intervals, evolved with experience and proved to be a highly successful way of achieving acceptable pressure-time courses. Decompression combined slow linear release of pressure with overnight halts for sleep. On one occasion a depth of 660 m s.w. was reached by breathing 40 kPa oxygen, 10% nitrogen, remainder helium. Throughout all exposures, teams of investigators followed the changes in cardiovascular, respiratory, haematological, neurophysiological and metabolic status, and mental performance of the volunteers. Some major findings were that the neurophysiological and behavioural changes could be assigned to the motor, or vestibular, or cerebral, or autonomic systems, and were mainly first observed during compression. The subjects suffered, apparently from severe nitrogen narcosis, when breathing 10% (by volume) nitrogen in oxygen-helium at 420 m s.w. Lung ventilation was remarkably adaptable to the oxygen requirements of exercise at all depths, but cardiac output was adversely affected at 540 m s.w., particularly for heavier workloads. Ventilatory responses to carbon dioxide were significantly elevated after diving. Thermal balance was seen to be precarious, but nevertheless it was achieved by the normal subjective assessments of comfort. Water loss was affected by diminished evaporation from the skin. Skin temperature sensitivity was changed and took many days after the dives to return to normal. Energy requirements increased for work purposes, but basal metabolic rate was undisturbed. Body chemistry altered at pressures in excess of 300 m s.w., for example thyroid hormone and nitrogen balances were affected. No decompression sickness was encountered until the pressures were low, but marked haematological changes could occur during decompression. Every change that occurred during these dives reverted to normal, mostly before the end of the decompression. It is concluded that diving with oxygen-helium breathing mixtures to depths as great as 540 m s.w. can be effective and safe. An attempt is made to assess the physiological significance of the principal findings.