We used mice deficient in expression of inducible NO synthase (iNOS /) to directly examine the role of iNOS in impaired vasoconstrictor responses following tumor necrosis factor- (TNF-). In iNOS +/+ mice, contraction of carotid arteries in response to prostaglandin F₂ (PGF₂) was impaired following TNF- (100 µg/kg ip)(n = 10, P < 0.01). In contrast to responses in wild-type mice, contraction to low concentrations of PGF₂ were normal, but maximum contraction to PGF₂ was impaired in arteries from iNOS / mice treated with TNF- [0.35 ± .0.02 g (n = 8) following vehicle and 0.25 ± 0.02 g (n = 7) following TNF- (P < 0.05)]. Aminoguanidine, a relatively selective inhibitor of iNOS, partially restored contraction to PGF₂ in vessels from iNOS +/+ mice but had no effect in iNOS / mice injected with TNF-, suggesting that a mechanism(s) other than iNOS contributes to impaired responses. In contrast to contractile responses, relaxation of the carotid artery in response to acetylcholine and nitroprusside was not altered following TNF- in iNOS +/+ or iNOS /mice. Responses of carotid arteries from iNOS / mice and effects of aminoguanidine suggest that both iNOS-dependent and iNOS-independent mechanisms contribute to impaired contractile responses following TNF-.