The goal of this research is to show the relationship between the duodenum morphology of laying hens (Gallus domesticus) of ISA Brown cross and typological peculiarities of autonomic tonus in the poultry organism. To achieve the goal a group of adult chickens (aged one year) was investigated to determine an aggregate tonus of their autonomic centers based on the study of heart rate variability (variation pulsometry). According to the results all the poultry was divided into two groups: sympatheticotonic chickens (hereinafter referred to as ST chickens) and sympathetico-normotonic chickens (hereinafter referred to as ST-NT chickens) The first group of poultry is characterized by a clear predominance of sympathetic centers tonus while within the second group there is a slightly increased tonus of parasympathetic centers. Using a set of various morphological methods a comparative analysis of the duodenum structure in these groups was made. The analysis revealed that ST chickens unlike ST-NT chickens are characterized by a higher value of the duodenum absolute length (by 1.6 cm) and its relative length (by 1.4 %) as well as lower values of its internal perimeter (by 0, 25 cm). Apart from this, an increase of absorption surface of the duodenum mucous membrane is observed in ST-NT chickens. Increase in villus height (by 143.7 micrometers) and their quantity (by 61.5) per 1sm 2 contribute greatly to it. In the duodenum muscle membrane the thickness of both layers has larger values under a distinct sympathetic tonus in ST-NT chickens. In a circular layer difference between poultry groups reaches 21 micrometers while in a longitudinal it approaches 4.9 micrometers. Investigating a connective tissue it was found that the relative area of its fibers near the mucosal crypts of ST chickens is larger by 0.21 % than that of the ST-NT chicken. In the muscle layer area a superiority of ST chickens according to this indicator reduces to 0.15%.