Systemic inflammation causes respiratory and circulatory dysfunctions. To study the mechanisms of these disorders, it is necessary to simulate the syndrome of systemic inflammatory response (SIRS) in an acute experiment on animals anesthetized with urethane. The aim of the present study was to test the hypothesis that one of the SIRS symptoms can be reproduced in these experimental conditions, namely — the change in the number and composition of blood leukocytes. The work was performed on laboratory rats (male Wistar, 250–300 grams) anesthetized with urethane (1600 mg/kg, i. p.) and divided into three groups. Animals of the first group (n = 5) underwent surgery simulating some surgical approaches used in the study of respiratory and circulatory functions. The second group (n = 5) was administered intravenously with 500 μg of bacterial lipopolysaccharide (LPS) dissolved in 1 ml of saline. In the third group (n = 5) the sham operation was combined with the introduction of LPS. Measurement of the number of white blood cells in blood samples taken from the caudal vein showed that the introduction of LPS in the second and third groups caused a decrease in the total number of white blood cells, mainly due to neutrophils. The obtained results prove that in an acute experiment in rats anesthetized with urethane, intravenous LPS can cause leucopenia, that is, reproduce one of the most important (along with tachycardia, tachypnea, and changes in body temperature) symptoms of SIRS.