New Zealand white rabbits were given an intravenous injection of lipopolysaccharide (LPS = 200 µg/Kg) from E. agglomerans. Four hours later, the animals were anesthetized, the lungs removed, and pulmonary arteries dissected and mounted in standard tissue baths for the measurement of vascular tone. Contractile responses to phenylephrine were significantly depressed in arteries from LPS exposed animals compared to controls. The L-arginine analog N(G-) nitro-L-arginine (LNA) restored contractions to near control levels. These results indicate that LPS administered in vivo may lead to the development of pulmonary hypotension by inducing the production of nitric oxide synthase in pulmonary vascular smooth muscle.