The content of corticosterone in the adrenals and blood plasma, as well as the weight of the adrenals and their histology, were investigated in 77 inbred rats (38 animals of the August line and 39 animals of the Wistar line). It was shown that after products of ascarid metabolism were administered to the experimental rats, the corticosterone content in the adrenals and blood plasma changed substantially. Despite the equal doses of the agent administered, statistically significant differences between lines were detected in the corticosterone indices in the blood plasma of the experimental rats, and they were also present in the control (healthy) animals. In the latter, no differences between lines were established in the corticosterone content in the adrenals, although in the experimental rats these differences arose at many periods of observation. It is concluded that the level of corticosterone content of the blood in the initial normal state, as well as the degree of its deviations from the initial norm, are determined (with equal conditions with respect to paratypic factors) by the genetic constitution (genotype) of the organism.