Previous studies have shown that the photoplethysmogram (PPG) may be a useful tool for the noninvasive detection of hypovolemia. The focus has been on determining if frequency analysis of the respiratory induced variations of the PPG can be used as an indicator of blood volume. In this preliminary study, we evaluate these frequency analysis techniques for two subjects undergoing Lower Body Negative Pressure (LBNP) induced hypovolemia. Using Matlab?-based software the power of the respiratory component and heart rate component were calculated using the periodogram method for spectral estimation. Consistent with other studies our algorithms were able to automatically detect changes in the respiratory variations. We found a significant increase in the respiratory variations in the PPG during simulated hypovolemia. By taking the ratio of the respiratory power to the heart rate power we consistently detected hypovolemia in subjects corresponding to sequestration of approximately 2 liters of blood (LBNP >70 mm Hg). The increase in this ratio occurred before significant change in blood pressure or tachycardia were observed.