Bathing in Japanese style may carry negative effects as water pressure on the chest and thermal stimulus on hemodynamics take place. We have explored the influence of bathing in high temperature water on the change of heart rate variability (HRV). Fourteen young healthy male adults, ageing in range from 28 to 42 years old (the average was 35.8 years old) were selected and took a hot water bath (38 and 41 degrees C) for 15 min long. Bathing in 38 degrees C water brought no significant change in heart rate (HR) and blood pressure (BP), and the HR in 41 degrees C increased in early stage. In HRV, high frequency (HF) power did not have significant change with little increase in early stages of bathing in 38 degrees C and decreased continuously in 41 degrees C. Low frequency (LF) power and very low frequency (VLF) power decreased gradually in later stages of bathing, but the degree of decrease was larger in 41 degrees C. In this study, data concerning dizziness after bathing at 41 degrees C was obtained (we named it as a "dizzy case"). HF and LF trends in this case followed the same pattern in comparison with others\' average, but the decrease was larger. Additionally, there was no increase in the LF/HF at later stage of bathing. It is thought that this reflects a decreased in autonomic nerve activity. In normal subjects the VLF increased in later stages of 38 and 41 degrees C bathing, but in the dizziness-experiencing subject, the increase was very significant. It is conceivable that this reflected excessive parasympathetic reflex. Except the dizzy case HF decreased continuously in later stage of bathing in both 38 and 41 degrees C, but VLF slightly increased. Recently there was an express opinion that the VLF correlates with the prognosis; therefore the change of VLF in this study is very interesting. Based upon the results of this study we propose that the optimum period of time for bathing in water 41 degrees C in temperature is 5 min or less, and that for water 38 degrees C in temperature is 10 min or less.