Inbred rats from the Maudsley Reactive (MR) strain had lower concentrations of norepinephrine in hypothalamus, heart, and spleen, and lower total catecholamines in adrenal than inbred Maudsley Non-Reactive rats (MNRA line). In contrast, they had a higher concentration of telencephalic NE than MNRA rats. These results confirmed previous findings obtained on rats maintained by non-systematic breeding within the two lines. Comparisons were also made between MR and a second Maudsley Non-Reactive strain (MNR). Rats of the two Non-Reactive lines (MNRA, MNR) have been bred from the same foundation population and selected for the same behavioral characteristics, but have been genetically isolated from each other for many generations. It was found that MR rats showed differences from MNR rats in hypothalamic and peripheral (but not telencephalic) NE concentrations similar to those seen in MR/MNRA comparisons. Since rats of the two Non-Reactive lines differ appreciably from MR rats in open-field defecation (the criterion on which they were selected), their differences from MR rats in a neurochemical system involved in sympathetic function suggests that this system may be functionally related to well established behavioral and physiological differences between the lines.