The porcine pulmonary vascular and airway responses to exogenous 5-hydroxytryptamine (5-HT), norepinephrine, prostaglandin F2 alpha (PGF2 alpha), and angiotensin II were evaluated before and after ketanserin, a 5-HT2 receptor antagonist. Ketanserin blocked the 5-HT-induced increases in airway and pulmonary artery pressures, whereas the increases in airway and pulmonary artery pressures caused by norepinephrine, PGF2 alpha, or angiotensin II were not significantly modified by ketanserin, indicating a relatively high degree of specificity for 5-HT2 receptors. The role of endogenous 5-HT in mediating endotoxin-induced respiratory failure was evaluated by treating pigs with ketanserin. Escherichia coli endotoxin (055-B5) was infused intravenously into anesthetized 10- to 14-wk-old pigs at 5 micrograms/kg the first h, followed by 2 micrograms/kg/h for 3.5 h. Ketanserin was infused at 300 micrograms/kg before endotoxin plus 67 micrograms/kg/h during endotoxemia. During Phase 1 (i.e., 0 to 2 h), the endotoxin-induced increases in pulmonary vascular resistance and room air alveolar-arterial oxygen difference and the decreased cardiac index and lung dynamic compliance were not significantly modified by ketanserin. However, during Phase 2 (i.e., 2 to 4.5 h) endotoxemia, ketanserin attenuated the endotoxin-induced pulmonary hypertension and the increases in pulmonary vascular resistance, alveolar dead space ventilation, and alveolar-arterial oxygen difference. Ketanserin also attenuated the Phase 2 bronchoconstriction and the decreased cardiac index, but did not modify the endotoxin-induced increase in alveolar-capillary permeability. These results indicate that 5-HT plays little or no role in mediating the early (i.e., less than 2 h) response to endotoxin.(ABSTRACT TRUNCATED AT 250 WORDS)