Examinations of different patients with duodenal ulcer performed during one year revealed seasonal changes in the function of the regulatory systems. Secretion of histamine, serotonin, insulin, and hydrocortisone was increased in spring and reduced in autumn. Secretion of adrenalin, noradrenalin and acetylcholinesterase activity were high in summer and low in autumn. The acid-producing function of the stomach did not depend on the season. It is suggested that deviations in the activity of the regulatory systems in spring and autumn may cause seasonal exacerbations of duodenal ulcer.