We investigated the responses of the frequency components of heart rate (HR) and blood pressure (BP) variabilities to progressive changes in autonomic activity induced by the graded head-up tilt technique in 12 normal subjects (age 19-27 yr) under the condition of frequency-controlled respiration (0.25 Hz). During low-level tilt (0-30 degrees), the R-R interval was unchanged and the amplitude of the high-frequency (HF; 0.25 Hz) component of HR variability showed only a slight insignificant decrease. The amplitude of the low-frequency (LF; 0.04-0.15 Hz) component of HR variability increased progressively as the angle increased (P < 0.05). During high-level tilt (30-90 degrees), the R-R interval and the HF amplitude of HR variability decreased progressively with tilt angle (P < 0.001 for both). The LF amplitude of HR variability peaked at a tilt angle of 30 degrees. The LF-to-HF ratio of HR variability and the LF amplitude of systolic and diastolic BP variabilities increased progressively as the tilt angle increased from 0 to 60 degrees (P < 0.001), although systolic and diastolic BPs were unchanged. These results suggest that mixed autonomic responses to orthostatic stress, which are thought to be mediated by both cardiopulmonary and arterial baroreflex mechanisms, can be distinguished by changes in the frequency components of HR and BP variabilities.