['Cytokines do not only mediate responses to infection, but are also involved in behavioral and physiological responses to psychological stressors. IL-6 has received special attention in relation to the development of posttraumatic stress disorders and depression.', 'We tested effects of prior injection of rat recombinant IL-6 (rrIL-6) on behaviors induced by the forced swim (FS) stressor, and central and peripheral responses of IL-6 to FS.', 'Rats were injected intraperitoneally with either rrIL-6 (250 ng/0.5 ml) or equal-volume sterile saline twice within an interval of 24 h. One hour after each injection, the rats were exposed to FS or remained at the home cage (control).', 'Injection of rrIL-6 did not affect immobility, swimming or climbing behaviors during FS compared with the saline control. Although FS was not a significant factor for hypothalamic and midbrain IL-6 mRNA and plasma IL-6 responses, FS with prior administration of rrIL-6 significantly increased hypothalamic IL-6 contents in response to FS compared with the saline injection-FS condition.'] Does prior intraperitoneal injection of rat recombinant IL-6 increase hypothalamic IL-6 contents in subsequent forced swim stressor in rats?