BACKGROUND AND OBJECTIVES: This study was designed to examine whether the multifractal Hurst exponents of heart rate can estimate activating and deactivating spontaneous baroreflex sensitivity (SBRS). SUBJECTS AND METHODS: Age and sex matched 24 healthy volunteers were placed in the supine position and head-up tilt position for 30 minutes, and the systolic blood pressure and heart rate were measured in an noninvasive manner. When the RR interval (RRI) sequence and systolic pressure sequence simultaneously increased or decreased for more than three successive beats, the slope of the linear regression line of systolic blood pressure and RR interval sequence was defined as the value of spontaneous baroreflex sensitivity (SBRS). From the RR intervals, very short-term (alpha range, < or =12 heartbeats), short-term (beta range, > or =12 heartbeats), and the multrifractal Hurst exponents were calculated to determine the linear correlation.coefficients for SBRS. RESULTS: In the supine position, the linear correlation coefficients (p) of SBRS and H-5alpha, H-4alpha, H-3alpha, H-2alpha, H-1alpha, H1alpha, H2alpha, H3alpha, H4alpha, H5alpha were -0.195 (NS), -0.207 (NS), -0.232 (NS), -0.282 (NS), -0.355 (NS), -0.621 (0.003), -0.650 (0.001), -0.677 (0.001), -0.699 (0.0006) and -0.708 (0.0005), respectively. In the tilting position, the linear correlation coefficients of SBRS and H-5beta, H-4beta, H-3beta, H-2beta, H-1beta, H1beta, H2beta, H3beta, H4beta, H5beta were 0.024 (NS), 0.020 (NS), 0.010 (NS), -0.028 (NS), -0.193 (NS), -0.627 (0.0034), -0.607 (0.0045), -0.598 (0.0053), -0.572 (0.0084) and -0.539 (0.0141), respectively. CONCLUSION: Some of the very short-term and short-term generalized Hurst exponents, such as H5alpha and H1beta, can be used for the estimation of spontaneous baroreflex sensitivity during patient placement in the supine and head-up tilt position, respectively.