Effects of domestication of Norway rats on their open-field behavior and the response of the pituitary-adrenal system were studied by comparing domesticated and wild gray rats that had been selected for 42 generations for reduction and maintenance of aggressive behavior toward humans. Basal corticosterone level and adrenal weight were also determined at the advanced selection. No significant differences in behavior (with the exception of rearing number) and corticosterone level were found between domesticated and wild rats on the first day of testing. The second encounter with a novel situation revealed differences in both behavioral and hormonal responses between 2 compared rat lines. The results obtained are considered from the viewpoint of their similarity to, or difference from, the data available in the literature on comparison of laboratory and wild rats in the open-field test. The difference between the laboratory rats and those selected by us for reduced aggressiveness seems to be due to the fact that breeding at this stage is done not only for reduced aggressiveness, but also for increased exploratory activity.