The effects of tannin purified from cotton bracts were studied on pulmonary arteries isolated from rabbit lungs. Tannin caused concentration-dependent contractions and maximal responses averaged 257 of the contraction induced by erogenous norepinephrine. Tannin-induced contractions were blocked by norepinephrine treatment of tissues with either indomethacin or SQ 29,548, a thromboxane receptor antagonist. Removal of the endothelium abolished the response to tannin. Confirmation of endothelial cell removal was obtained by comparing the inhibitory response to a low concentration of acetylcholine In control versus test tissues. We conclude that tannin contracts the smooth muscle of rabbit pulmonary arteries by stimulating the production and release of thromboxane A(2) from pulmonary endothelial cells.