The major pore-forming exotoxin of Staphylococcus aureus, staphylococcal α-toxin, causes thromboxane-mediated pulmonary hypertension and prostanoid-independent protracted vascular leakage in perfused rabbit lungs (8, 9). We asked whether lung responsiveness to the staphylococcal agent would be altered by a preceding period of endotoxin priming. Isolated rabbit lungs were perfused with Krebs-Henseleit buffer in the presence or absence of 100 ng/ml Salmonella abortus equii endotoxin for up to 5 h. The lipopolysaccharide exposure evoked the release of large quantities of tumor necrosis factor into the vascular and alveolar spaces but did not significantly alter pulmonary artery pressure, organ weight, or the repeatedly assessed capillary filtration coefficient (Kfc). Two and 4 h after endotoxin administration, α-toxin (10 to 30 ng/ml) was bolus-injected into the pulmonary artery. Toxin-evoked prostanoid generation (TxB2, 6-keto-PGF1α) and pressor responses were markedly accelerated and enhanced in endotoxin-primed lungs, both for the 2 h and the 4 h priming period. No significant influence of endotoxin was noted when applied simultaneously with α-toxin. Cyclooxygenase inhibition suppressed the α-toxin-evoked pressure rise in both endotoxin-primed and nonprimed lungs. Endotoxin priming did not influence the α-toxin-induced protracted increase in Kfc values, assessed in the presence of cyclooxygenase inhibition. We conclude that endotoxin primes rabbit lungs for enhanced prostanoid generation and pulmonary hypertension in response to S. aureus α-toxin. Such cooperativity of endotoxin primrng and exotoxin triggering may be relevant in critically ill patients suffering from both endotoxemia and gram-positive sepsis.