Various essential oils are rich in carvacrol, a monoterpenic phenol isomeric with thymol. This study was undertaken to assess the vasorelaxant effects of thymol and carvacrol in rat isolated aorta and the putative mechanisms underlying these effects. Thymol and carvacrol produced a concentration-dependent relaxation on the aortic ring preparations pre-contracted using KCl (IC(50) value of 64.40 +/- 4.41 and 78.80 +/- 11.91 microm, respectively) or using phenylephrine (PHE, 0.1 microm) (IC(50) value of 106.40 +/- 11.37 and 145.40 +/- 6.07 microm, respectively) and inhibited the concentration-response curves of aortic rings to PHE or KCl. In Ca(2+)-free medium with ethylene glycol-bis(2-aminoethylether)-N,N,N',N'-tetraacetic acid (2 mm), thymol and carvacrol both at 1000 microm completely abolished the phasic component of PHE-induced endothelium-containing ring contractions. At 400 microm, thymol and carvacrol significantly reduced the CaCl(2)-induced contractions in Ca(2+)-free medium. Furthermore, both thymol and carvacrol (300 and 1000 microm) significantly reduced the contraction evoked by phorbol dibutyrate (1 microm), an activator of protein kinase C. Magnitude of this inhibitory effect was enhanced in the presence of the Ca2+ pump inhibitor, thapsigargin (1 microm). At 1000 microm, neither thymol nor carvacrol altered the resting potential of vascular smooth muscle cells. In conclusion, thymol and carvacrol induced an endothelium-independent relaxation in rat isolated aorta, an effect that seems mediated through some mechanisms probably involving a transduction pathway between Ca(2+) release from sarcoplasmic reticulum and/or regulation of the Ca2+ sensitivity of the contractile system. Moreover, it's conceivable that thymol and carvacrol, at low concentrations, block the Ca(2+) influx through the membrane.