The lymphocyte response to Con A and IL-2 synthesis was clearly suppressed in rats after thermal injury. In burn rats, the cAMP content in lymphocytes was significantly increased, but cGMP level was not changed. The serum of burn rat could inhibit Con A-induced lymphocyte proliferation and IL-2 production. The burn rat serum could also increase intracellular cAMP concentration. Dynorphin A enhanced lymphocyte proliferation and IL-2 synthesis, increased cAMP level in lymphocytes. Naloxone could inhibit the action of dynorphin A. Moreover, ynorphin A could partly reversed suppression effect of burn serum on lymphocyte proliferation and suppressed the increase of intracellular cAMP content induced by burn and rat serum. Our results suggested that dynorphin A augmented immune function of burn rats, and the action of dynorphin A was mediated by cAMP pathway.