The purpose of this study was to make clear the relaxative effects of supine floating (SF) on heart rate, blood pressure and cardiac autonomic nervous activity in males. Ten males served as subjects (n=10, mean age: 22.4 yrs). All subjects gave informed consent before participating. Water and room temperature were 30 degrees C. Heart rate and blood pressure were measured during SF or control (C) conditions. Cardiac autonomic nerve activity was estimated with the power spectrum analysis of heart rate variability (HRV) by using the Fast Fourier Transformation (FFT). High frequency (HF; 0.15-0.40 Hz) and the ratio of low frequency (LF; 0.04-0.15 Hz) to HF (LF/HF) were used as an indicator of cardiac vagal activity and sympatho-vagal balance, respectively. Those values were showed logarithmically (LogHF and LogLF/LogHF). LogHF during SF condition was significantly increased, LogLF/LogHF, heart rate and blood pressure were significantly decreased. These data indicate that cardiac vagal activity is enhanced and sympathetic nervous activity is suppressed by reciprocal response.