The identity of the histamine-potentiating activity detected in the rat anaphylactic pleural washing was investigated. Wistar rats of both sexes, weighing 150-200 g, were sensitized by injecting subcutaneously (sc) a mixture of ovalbumin and Al(OH)3 14 days before allergen challenge. In sensitized rats, intrapleural (ipl) injection of ovalbumin (12 micrograms/cavity) caused an intense protein exudation. A single ipl administration of compound 48/80 (12 micrograms/cavity) exhausted the resident mast cell population and turned the pleural cavity hyporeactive to the allergen challenge performed 5 days later. Allergen-induced exudation occurred in parallel to a dramatic decrease in the amount of cell-stored histamine (from 9.6 +/- 1.4 (N = 8) to 1.3 +/- 0.1 (N = 6) micrograms/cavity, P < 0.001) in the pleural fluid within 10 min. The anaphylactic cell-free pleural washing obtained at this time, as well as histamine at a concentration equivalent to that stored in pleural mast cells (10 micrograms/cavity), did not induce pleural exudation when injected into normal rats. In contrast, the combined administration of histamine and anaphylactic pleural washing led to remarkable pleural exudation, comparable to that obtained with a high dose of histamine (200 micrograms/cavity) alone. It is noteworthy that the anaphylactic washing from compound 48/80-pretreated rats failed to synergize with histamine. Also, synergism was not reproduced when recipient rats were pretreated with methysergide (50 micrograms/cavity). Consistently, serotonin (5 micrograms/cavity) acted synergistically with histamine (10 micrograms/cavity), producing a greater exudative response than observed with the sum of the effects of each vasoactive amine alone. The results indicate that serotonin accounts for the histamine-potentiating activity noted in the anaphylactic pleural washing, confirming that the synergistic interaction between these vasoactive amines plays a critical role in the rat allergic pleurisy.