Recently, some studies have demonstrated enhanced cardiac vagal nerve activity during the right lateral decubitus position in patients with chronic heart failure and coronary artery disease (Fujita et al, 2000 and Yang et al, 2008). However, there has been little research on the autonomic nervous system and circulation in the lateral decubitus position for normal healthy volunteers. In the present study, we compared the impact of the left and right lateral decubitus positions and the supine position on heart rate variability (HRV), including circulatory regulation. We performed electrocardiographic measurements in 58 healthy volunteers aged 23.0 ± 3.2 (mean ± SD) years. Further, a skilled cardiologist conducted echocardiographic assessment of the inferior vena cava (IVC) in each posture for 3 of 58 volunteers. A male patient with situs inversus totalis (63 years) also participated in this study. Subjects randomly maintained each lateral position at an angle of over 45° for 10 min after remaining in the supine position. Postural change was performed manually by two trained researchers. The measured data is represented as mean ± SEM. A value of p < 0.05 is considered statistically significant. No statistically significant differences were observed either in low frequency (LF) or high frequency (HF) HRV or in their ratio (LF/HF) using multiple comparison among three positions. However, the heart rate (HR) was significantly lower in the left lateral position than in the supine position (61.5 ± 0.9 bpm vs. 64.7 ± 1.1 bpm, respectively, p < 0.05, multiple comparison: Dunnet). The shape of the IVC in the supine position appeared oval in echocardiogram, and became more like a plane and a circle while in the left and right lateral decubitus positions, respectively. Both the diameter and area of IVC decreased, and hence, blood flow in IVC became more rapid in the left lateral position compared to the supine position. The echocardiographic evaluation also revealed that the two lateral decubitus positions caused different changes in circulation in the IVC. Unlike the 58 healthy subjects, the patient showed a decrease in HR in the right lateral position compared with the supine position (52.2 ± 0.3 bpm vs. 55.7 ± 1.5 bpm, respectively, p < 0.05, paired t-test). The HRV findings suggest that the two recumbent positions have no influence on the autonomic nervous system in healthy volunteers. However, the reduction in HR in the left lateral position may be affected by anatomical changes, especially in the shape of the IVC.