To evaluate whether low-pressure baroreceptors located in the right atrium could affect the heart rate (HR) during spinal anesthesia, the authors determined the effects of right atrial pressure changes associated with body tilt on HR in 40 unpremedicated patients. Ten-degree head-up body tilt produced significant increases in HR of 6 +/- 1 and 6 +/- 1 beats/min (mean +/- SE, P less than 0.01) and significant decreases in systolic arterial pressure of 2.8 +/- 0.9 and 6.6 +/- 1.7 mm Hg (P less than 0.01) during low (T-10 +/- 0.2, n = 20) and high (T-4 +/- 0.2, n = 20) analgesic levels of tetracaine spinal anesthesia, respectively. Ten-degree head-down body tilt caused significant decreases in HR without significant changes in systolic arterial pressure during spinal anesthesia. The reflex HR responses to body tilt were similar between low and high levels of spinal anesthesia and were preserved after administration of sedatives. The magnitudes of changes in right atrial pressure associated with body tilt were similar during spinal anesthesia and after sedation. These findings suggest that HR responses to head-up body tilt are mediated mainly by arterial baroreceptors even in the face of decreased venous return during low or high levels of spinal anesthesia and that light sedation does not impair this reflex HR response.