The psycho-physiological effects of several sound stimulations were investigated to evaluate the relationship between a psychological parameter, such as subjective perception, and a physiological parameter, such as the heart rate variability (HRV). Eight female students aged 21-22 years old were tested. Electrocardiogram (ECG) and the movement of the chest-wall for estimating respiratory rate were recorded during three different sound stimulations; (1) music provided by a synethesizer (condition A); (2) birds twitters (condition B); and (3) mechanical sounds (condition C). The percentage power of the low-frequency (LF; 0.05 < or = 0.15 Hz) and high-frequency (HF; 0.15 < or = 0.40 Hz) components in the HRV (LF%, HF%) were assessed by a frequency analysis of time-series data for 5 min obtained from R-R intervals in the ECG. Quantitative assessment of subjective perception was also described by a visual analog scale (VAS). The HF% and VAS value for comfort in C were significantly lower than in either A and/or B. The respiratory rate and VAS value for awakening in C were significantly higher than in A and/or B. There was a significant correlation between the HF% and the value of the VAS, and between the respiratory rate and the value of the VAS. These results indicate that mechanical sounds similar to C inhibit the para-sympathetic nervous system and promote a feeling that is unpleasant but alert, also suggesting that the HRV reflects subjective perception.