Despite the great value of diagnostic bloodwork for identifying disease in animals, the volume of blood required for these analyses limits its use in laboratory mice, particularly when they are clinically ill. We sought to determine the effects of acute blood loss (ABL) following blood collection for diagnostic bloodwork in healthy mice compared with streptozotocin-induced diabetic and dextran sulfate sodium (DSS)-treated dehydrated mice. ABL caused several mild changes in the control mice, with significant decreases in body weight, temperature, and activity in both experimental groups; increased dehydration and azotemia in the DSS-treated mice; and a significant drop in the blood pressure of the diabetic mice. To determine whether these negative outcomes could be ameliorated, we treated mice with intraperitoneal lactated Ringers solution either immediately after or 30 min before ABL. Notably, preABL administration of fluids helped prevent the worsening of the dehydration and azotemia in the DSS-treated mice and the changes in blood pressure in the diabetic mice. However, fluid administration provided no benefit in control of blood pressure when administered after ABL in the diabetic mice. Furthermore, fluid therapy did not prevent ABL-induced drops in body weight and activity. Although one mouse not receiving fluid therapy became moribund at the 24-h time point, no animals died during the 24-h study. This investigation demonstrates that blood for diagnostic bloodwork can be collected safely from clinically ill mice and that preemptive fluid therapy mitigates some of the negative changes associated with this blood loss.