BACKGROUND: We designed this study to compare acid-base parameter with hemodynamic parameter as the predictor of acute blood loss. And we tried to evaluate the changes of electrolyte and to correlate the change of each parameter with that of cardiac output (CO) during acute blood loss. METHODS: Seven anesthetized (isoflurane 1.0-1.5%), paralyzed and mechanically constant ventilated dogs submitted to hemorrhage were studied. The dogs were hemorrhaged by progressive withdrawal of 50% of blood volume. After withdrawal of each 10% of blood volume mean arterial blood pressure (MAP), CO, arterial blood gas analysis, electrolytes and hematocrit (Hct) were measured. RESULTS: MAP, pH, PaCO2, PaO2 and base excess (BE) were decreased significantly according to hemorrhage and correlated with the percent decrease in CO. Serum sodium concentration and calcium concentration were not changed. Serum potassium concentration was increased and Hct was decreased significantly and correlated with the percent decrease in CO. Correlation between BE and percent decrease in CO (r=0.771) was superior to the correlations between any other parameter and percent decrease in CO. CONCLUSION: More negative BE is the best indicator of ongoing hemorrhage. Thus BE may be the best laboratory monitor of hemorrhage and the evaluation of acid-base state and the changes of serum potassium concentration are important in the acute hemorrhagic situation.