We measured β-endorphin (BE) and β-lipotropin (BLPH) plasma concentrations (by means of an HPLC-RIA coupled method) during attacks as well as during symptom-free periods in a group of 28 patients with immunochemical (21) or functional (7) C1 inhibitor deficiency. Thirteen patients suffering from chronic urticaria served as controls. Three orders of considerations prompted us to initiate the present study: the clinical relationship between stress and the onset of acute episodes, the possible effects of repeated stressful situations, as are the attacks themselves, on the patients' neuroendocrine system and the well-known existence of close links between the immune system and endogenous opioids. The results show that plasma BE (and, to a lesser extent, BLPH) is dramatically increased during the attacks. In symptom-free periods many patients show very high BE concentrations, often in the presence of slightly elevated concentrations of BLPH and of ACTH. These observations suggest that patients with hereditary angioneurotic edema show a modified pro-opiomelanocortin-synthesizing cell activity that can result in a massive release of BE from the readily disposable pool present in the pituitary and/or an increase in the turnover of the peptide as evaluated by the BLPH/BE ratio.