Lithium chloride (LiCl) is used for treatment of viral diseases, anxiety, depressive maniac disorder and experimental allergic encephalomyelitis. Lithium has also been shown to exert an effect on elements of the immune system. The aim of this work was to study the effect of a single dose of LiCl on the anaphylactic sensitized response in vitro from guinea pig aortic rings. All guinea pigs were sensitized with a single subcutaneous injection of 10 mg/ml of hen egg albumin (EA) dissolved in saline solution with and without Freund's adjuvant. The experimental group received a single dose of LiCl (2 or 4 mg/kg i.p.) on the same day of the immunization. Thoracic aortic rings without endothelium were obtained from each guinea pig and isometric contraction was recorded by suspending the preparations in tissue chambers. The anaphylactic response was defined as the contraction occurring after the addition of EA to the tissue chamber (100 microg/ml) and is expressed normalized to the norepinephrine-induced (1 microg/ml) contraction for each tissue. In the groups with LiCl the sensitized response was decreased 20-41% compared to control. In animals treated with Freund's adjuvant, the control group had an increased response (54.8%). In addition, for the lithium treated animals, the sensitized response was greater in the animals which had been given the adjuvant (41% vs. 31% for animals without adjuvant). Our results show that the single dose of LiCl simultaneously with the EA immunization suppressed the sensitized response in aortic rings from male guinea pigs. LiCl did not exert any adjuvant effect on the response. Lithium may have profound immunomodulatory effects in animal models as well as in humans. LiCl's effect to modify the sensitized response may be due to reduced production of antibodies or by a difference in fixation of antibodies in the cell membrane.