The metabolism of endogenous arachidonic acid by mouse resident peritoneal macrophages infected in vitro with Toxoplasma gondii was studied. Prelabeling of macrophages with [5,6,8,9,11,12,14,15-3H]arachidonic acid and challenge with tachyzoites for 15 min resulted in a high mobilization of free labeled arachidonic acid (178%) in the culture medium. The parasites also triggered the synthesis of 6-keto-prostaglandin F1 alpha (47%), prostaglandin E2 (44%), leukotrienes C4 and D4 (33%) and 5-, 12-hydroxyeicosatetraenoic acids (155%). The study indicated that during the intracellular development phase of the parasites, 6-keto-prostaglandin F1 alpha (38%), prostaglandin E2 (31%) leukotrienes C4 and D4 (15%), hydroxyeicosatetraenoic acids (43%), and free arachidonic acid (110%) were secreted into the culture medium. Pretreatment of tachyzoites with phospholipase A2 inhibitors (4-p-bromophenacyl bromide and quinacrine) and no calcium in the culture medium resulted in inhibition of tachyzoite penetration into the macrophages and a decrease of the arachidonic acid metabolism. The triggering of the arachidonic acid cascade by T. gondii was dependent on the active penetration of the parasites into the macrophages, whereas preincubation of the macrophages with phospholipase A2 inhibitors did not affect penetration or free arachidonic acid release, thereby supporting a role for parasite phospholipase in the penetration process and in arachidonic acid mobilization from macrophage membrane phospholipids. Moreover, treatment of macrophages with phospholipase A2 inhibitors decreased the activities of the cyclooxygenase and lipoxygenase pathways, also suggesting an activation of host cell phospholipase A2 by the parasite.