AbstractDHA is abundant in the brain. Deficiency of DHA changes behavior in animals. The purpose of the present studies was to clarify the effect of DHA intake on hostility and plasma catecholamines. In study 1, forty‐one students took either DHA‐rich oil capsules containing 1.5–1.8 g DHA/d (17 females and 5 males) or control oil capsules containing 97% males) for 3 mon in a double blind fashion. They took a psychological test (P‐F Study) at the start and end of the study. Study 1 started at the end of summer vacation and ended in the middle of mental stress of final exams. In the control group, hostility measured by P‐F Study was significantly increased at the end of the study as compared with that measured at the start (+58%) was not significantly changed in the DHA group (‐14%) double blind two‐mon study (study 2), we measured plasma catecholamines and cortisol of students (3 females and 4 males for the DHA group and the same numbers for the control) at the start and end of the study. In study 2 the students were under a continuous stress of final exams that lasted for two mon throughout the whole study period. The plasma cortisol did not change in either group, but the norepinephrine concentration was significantly decreased in the DHA group (‐31%) group. These effects of DHA intake may be applied to people under psychological stress.