The vasoactive properties of 15(S)-hydroperoxyeicosatetraenoic acid (15-HPETE) and 15(S)-hydroxyeicosatetraenoic acid (15-HETE) were characterized in aortic rings of guinea pig (GPA), rat (RA) and rabbit (RbA), as well as pulmonary arteries from guinea pigs (GPPA) and humans (HPA). Four distinct patterns of activity were identified: 1) Cyclooxygenase-dependent relaxation (in GPA, GPPA and HPA). This could be speculated to be due to release of prostaglandin I2 or to conversion of 15-H(P)ETE to another vasorelaxant eicosanoid(s). The endothelium was the main source of this activity in GPA but not in HPA. 2) Cyclooxygenase-independent relaxation mediated by both endothelium and the smooth muscle proper (only in RA). 3) Endothelium-dependent contraction associated with the release of unknown factor(s) (in GPA, GPPA and HPA). 4) Endothelium-independent contraction (in RbA). Nitric oxide was not involved in the relaxation of GPA and RA, nor was endothelin in the contraction of GPA. 15-HPETE and 15-HETE always elicited analogous responses in the same preparations, probably because of rapid metabolism of 15-HPETE into 15-HETE or, even more likely, because both eicosanoids have identical modes of action. We concluded that depending on factors such as the species, the dose of the compounds and the presence of other vasoregulators, the overall response to 15-HPETE or 15-HETE may be vasodilation or vasoconstriction. In addition, the type of responses elicited with 15-HPETE and 15-HETE in RbA and RA differed conspicuously from those expressed in GPA, GPPA and HPA.