To assess the significance of stroke volume variation and intrathoracic blood volume index on the responsiveness to volume loading in mechanically ventilated canines with hemorrhagic shock．Methods：Healthy mongrel canines were studied first to standardize the modified Wiggers’blood loss shock method．The MAP(mean artery pressure)reached 50 mmHg and had maintained for 60 minutes．Graded volume loading had been performed with each volume loading step(VLS)consisting of 7ml.Kg-1 Ringers’given in 2 minutes．The sanle VLS was preformed after a stable period of 15 minutes again．Successive responsive VLSs were performed(increase in SV>5％after VLS)until continuous change in SV<5％(unresponsivel was reached．The values of HR(heart rate)，MAP,CVP(central venous pressure)，PAWP(pulmonary artery wedge pressure)。ITBVI(intrathoracic blood volume index)，and SVV(stroke volume variation)were determined before and 5 minutes after volume loading immediately．The responsive and unresponsive individuals were identified as two groups．Results：(1)Responsiveness after VLS：Fourteen canines were studied and a total of 134 VLSs were performed．In 94 VLSs，an increase in SV of more than 5％was reached．In 40 VLSs，an increase in SV of less than 5％was reached(unresponsive)．(2)The comparison of the pre—VLS values of hemodynamic variables between two groups：The pre．VLS values of MAP,1TBVI(79±27 mmHg，569±341 ml/m2)in responsive group were less than unresponsive group(98±15 mmHg，7844-407 ml/m2)，The pre·VLS values of SVV(14.5± 4.O％)in responsive group were more than unresponsive group(9.04±2.7％)，(P<0.05)，but no difference in the value of HR，CVP or PAWP between two groups．(3)The comparison of the changes in the values of hemodynamic variables after VLS between two groups：The values of △HR，△CVP,△PAWP,△ITBVI and △SVV in responsive group(-10.74±4.1bpm，0.34±0.1 mmHg，0.64±0.4 mmHg，55±47 ml/m2，-3.24±2.3％respectively)differed with those of unresponsive group-5.6±3.3 bpm，4.5±2.1 mmHg，5.4±2.4 mmHg，-40±99ml/m2，-0.2±0.9％respectively)，(P<0.05)．(4)The correlation between the pre—VLS values of hemodynamic variables and the change in SV after VLS(△SV)：Statistically significant correlations were found between △SV after VLS and the values of ITBVI，SVV before fluid loading(r=0.356，0.531 respectively)(P<0.05)．No correlation was found between △SV and the value of HR，MAP,CVP or PAWP before fluid loading(P>0.05)．(5)The correlation between the values of hemodynamic variables after VLS and △SV：Statistically significant correlations were also found between △SV and △CVP,△PAWP,△ITBVI，△SVV after fluid loading(r=-0.371，-0.448，0.438 and-0.376 respectively)(P<0.05)．No correlation was found between △SV and the value of△HR，△MAP after fluid loading(P>0.05)．(6)The assessment on the responsiveness to volume loading：By Using receiver operating characteristic analysis，the areas under the curve were 0.872 for SVV,and 0.689 for ITBVI，statistically more than those of HR，MAP,CVE PAWE As SVV value of 9.5％or more，will predict an increase in the SV of at least 5％in response to a VLS with a sensitivity of 92.6％and a specificity of 62.5％．Conclusion：SVV and ITBVI on the Assessments of responsiveness to volume loading are more useful indicators than HR．MAP,CVP,and PAWP. SVV was superior to ITBVI at predicting fluid responsiveness．SVV As a functional preload parameter Can be monitored online and may help to improve the hemodynamic management.