In burn injury, elevated sympathetic drive, probably in concert with elevated secretion of cortisol and glucagon, appears to mediate resting hypermetabolism. Elevated plasma catecholamine and corlisol and suppressed thyroid hormone concentrations provide a potentially powerful immunosuppressive hormonal milieu after thermal injury, but whether active intervention to modify these hormonal changes can restore immunocompetence in burn patients is not known. Hormonal systems that usually act to retain salt and water (renin, aldosterone, vasopressin) appear to be set at higher levels, and this does not appear to be associated with a demonstrable deficit of effective circulating volume. In human beings and hamsters, burn induced suppression of the thyroid and reproductive systems does not result from activation of the pineal gland, which itself shows some suppression of melatonin synthesis in burned hamsters. Sympathetic control of melatonin secretion by the pineal gland is partitioned from general sympathetic activity, and melatonin is not an index of general sympathetic activity.