<jats:p> <jats:list list-type="explicit-label"> <jats:list-item><jats:p>The objective of the present study was to investigate the mechanism of the relaxant activity of marrubenol, a diterpenoid extracted from <jats:italic>Marrubium vulgare</jats:italic>. In rat aorta, marrubenol was a more potent inhibitor of the contraction evoked by 100 m<jats:sc>M</jats:sc> KCl (IC<jats:sub>50</jats:sub>: 11.8±0.3 <jats:italic>μ</jats:italic><jats:sc>M</jats:sc>, maximum relaxation: 93±0.6%) than of the contraction evoked by noradrenaline (maximum relaxation: 30±1.5%).</jats:list-item> <jats:list-item><jats:p>In fura‐2‐loaded aorta, marrubenol simultaneously inhibited the Ca<jats:sup>2+</jats:sup> signal and the contraction evoked by 100 m<jats:sc>M</jats:sc> KCl, and decreased the quenching rate of fura‐2 fluorescence by Mn<jats:sup>2+</jats:sup>.</jats:list-item> <jats:list-item><jats:p>Patch‐clamp data obtained in aortic smooth muscle cells (A7r5) indicated that marrubenol inhibited Ba<jats:sup>2+</jats:sup> inward current in a voltage‐dependent manner (<jats:italic>K</jats:italic><jats:sub>D</jats:sub>: 8±2 and 40±6 <jats:italic>μ</jats:italic><jats:sc>M</jats:sc> at holding potentials of −50 and −100 mV, respectively).</jats:list-item> <jats:list-item><jats:p>These results showed that marrubenol inhibits smooth muscle contraction by blocking L‐type calcium channels.</jats:list-item> </jats:list> <jats:p><jats:italic>British Journal of Pharmacology</jats:italic> (2003) <jats:bold>140</jats:bold>, 1211–1216. doi:<jats:ext-link xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" xlink:href="10.1038/sj.bjp.0705561">10.1038/sj.bjp.0705561</jats:ext-link>