Prostacyclin (PGI2) is a potent endogenous vasodilator thought to act primarily through a cAMP-dependent mechanism. Vasorelaxation induced by either PGI2 or the stable analogues, iloprost and cicaprost is associated with a rise in cAMP levels, a response which can be mimicked by forskolin (direct activator of adenylyl cyclase) and potentiated by phosphodiesterase (PDE) inhibitors. There have, however, been several studies disputing a role for cAMP in mediating PGI2-evoked vasorelaxation. Moreover, increasing evidence suggests that PGI2 and its analogues act in part through the activation of K+ channels. To determine the role of cAMP and channels in these responses, the mechanisms of iloprost- or cicaprost-evoked vasorelaxation in guinea-pig aorta was investigated. Iloprost or cicaprost induced a concentration-dependent relaxation of phenylephrine-induced contractions, a response which was also mimicked by NECA and isoprenaline (Gs-coupled receptor agonists), forskolin and dibutyryl cAMP (cAMP analogue) and potentiated by Ro 20, 1724 (PDE inhibitor). However, pre-treatment with the adenylyl cyclase inhibitor, SQ 22536 and the protein kinase A (PKA) inhibitor, H89 failed to affect responses to iloprost or cicaprost. SQ 22536 also had no effect on responses to any vasorelaxant investigated in this tissue. However, in parallel experiments, measurement of cAMP levels revealed that that SQ 22536 completely inhibits the accumulation of cAMP induced by either iloprost or cicaprost. The involvement of channels in the response to iloprost or cicaprost was confirmed by the observation that inhibitors of large conductance, Ca2+ activated K+(BKCa) channels significantly attenuated vasorelaxation. Moreover, cicaprost activated BKCa channels in isolated cell-attached patches, although the response was unaffected by SQ 22536 but abolished by H89. In conclusion, iloprost and cicaprost induce vasorelaxation in guinea-pig aorta via a cAMP- independent mechanism that involves activation of BKCa channels. The discrepancy between functional changes in tension without detectable cAMP accumulation warrants further investigation.