<p>The article presents the results of the determination of the Kernohan index in vessels of the spleen at cold-blooded animals. The index was calculated for the arteries of the white pulp, in particular, the central artery of a lymphoid nodule and lymphoid sheaths near the vessels. This selection is based on the performance of white pulp filtration and immune functions. The study of the spleen conducted in the phase of morphological and functional maturity of the organ in the wells catfish (age 2 years), the lake frogs (age 24-26 months), and the green lizards (aged 23-25 months). The slices of material were fixed in 10-12 % chilled neutral formalin solution, liquid Carnoy, for microscopic studies; then they were embedded in paraffin. Histological sections have a thickness not exceeding 6 micrometers. Then there are dyeing by hematoxylin and eosin according to Van-Gison technology. Morphometric study of the spleen was performed under light microscopy using a microscope Biolam-Lomo and Micros MC-50. The microphotography of histological preparations was carried out with CAM V 200 video camera. It is mounted on a microscope Micros MC-50. The Kernohan index belongs to the indices which characterizing the functional immunomorphology in the spleen and is used to develop test criteria on in biomonitoring.</p><p>The wall thickness of the central artery lymphoid nodule is the smallest in the fish (3.4±1.11 micrometers) and the largest in the frog (20.47±4.46 micrometers), of a lizard, it is of 6.96±of 1.62 micrometers. The inner diameter is the smallest in lizards (4.32±2.44 micrometers) and the largest –in the frog (21.03±3.76 micrometers), in the fish is of 6.48±2.31 micrometers. The frog wall thickness of the central artery of lymphoid nodule and arteries lymphoid sheaths near the vessels are the highest (11.25±1.25 micrometers and 31.25±6.25 micrometers, respectively) among the studied animals. The fish`s wall thickness of inner diameter of the artery and arteries lymphoid sheaths near the vessels is 6.25±1.12 micrometers and 16.01±2.55 micrometers; in the lizards – 6.22±1.06 micrometers and was 3.56±0.53 micrometers, respectively. The Kernohan index of arteries lymphoid nodule is the larger than arteries lymphoid sheaths near the vessels by 1.35 times (in fish), by 2.69 times (in frog), and less than by 1.08 times (in lizard). The highest value of Kernohan index was registered for lizard: 3.2±0.61 of artery lymphoid nodule and 3.49±0.59 for arteries lymphoid sheaths near the vessels.</p>