Ten laboratory rats were experimentally invaded per os with the Eustrongylides excisus (Jäegerskiold, 1909) nematode larvae (L3–L4) received from Perca fluviatialis (Linnaeus, 1758), caught in the water area of the Dnipro-Buh Estuary, Ukraine. The aim of the experimental study was to measure the pathogenic effect of the given parasite on the mammals’ organisms by means of the histological analysis of the affected organs of the infected rats. The researched animals were divided into two groups. The experiment lasted 15 days; the second group was a control group. The histological analysis showed purulent-serous and purulent-fibrinous peritonitis, acute circulatory disorders in brain tissues (signs of hypoxic-ischemic lesions of the brainstem in the form of neuronal changes) and the heart (partial spasm of arteries and edema of interstitial myocardial tissue); pathological changes in the lungs (acute interstitial purulent pneumonia, acute focal emphysema of the lungs), liver (acute exudative hepatitis), kidneys (signs of shock reaction) and intestines (acute serous-purulent enteritis). Formation of non-specific granulomas was noticed, which contained the fragments of the parasitic larvae. The acute course of the purulent-serous and purulent-fibrinous inflammatory processes in the organs of the invaded rats was noticed; signs of systemic inflammatory response syndrome with pronounced distributive vascular leukocytosis in the studied organs.