The systolic pressure variation (SPV), which is the difference between the maximal and minimal values of the systolic blood pressure (SBP) after one positive-pressure breath, was studied in ventilated dogs subjected to hypotension. Mean arterial pressure was decreased to 50 mm Hg for 30 minutes either by hemorrhage (HEM, n = 7) or by continuous infusion of sodium nitroprusside (SNP, n = 7). During HEM-induced hypotension the cardiac output was significantly lower and systemic vascular resistance higher compared with that in the SNP group. The systemic, central venous, pulmonary capillary wedge pressure, and heart rates, were similar in the two groups. Analysis of the respiratory changes in the arterial pressure waveform enabled differentiation between the two groups. The SPV during hypotension was 15.7 ± 6.7 mm Hg in the HEM group, compared with 9.1 ± 2.0 mm Hg in the SNP group (P < 0.02). The Δdown, which is the measure of decrease of SBP after a mechanical breath, was 20.3 ± 8.4 and 10.1 ± 3.8 mm Hg in the HEM and SNP groups, respectively, during hypotension (P < 0.02). It is concluded that increases in the SPV and the Δdown are characteristic of a hypotensive state due to a predominant decrease in preload. They are thus more important during absolute hypovolemia than during deliberate hypotension.