Autonomic nerve functions under severe hyperbaric pressure were evaluated by measuring heart rate variability (HRV) and catecholamine excretion rate in 16 normal volunteers in submarine experimental facilities simulating conditions 330 m below sea level. HRV and urinary catecholamine levels were evaluated to assess sympathetic and parasympathetic tone. High-frequency HRV increased from 5.6 +/- 1.3 to 6.3 +/- 1.4 ms2 (p < 0.05), and SD of the average normal R to R intervals for 5-minute index (SDNNI) (time domain HRV parameter) increased from 77.2 +/- 32.7 to 93 +/- 33.8 ms (p < 0.05) after 3 days. Adrenaline/creatinine increased by 18% from a basal value of 4.04 +/- 0.44 ng/dL/h. Also, there was significant negative correlation between high-frequency and urinary cathecholamine levels. Evaluation of autonomic nerve functions under hyperbaric conditions by measuring HRV was shown to be a useful method. Thus, the present results indicate that the autonomic nerve functions of people who work under deep-sea conditions can be evaluated adequately by measuring HRV.