<jats:p>The <jats:italic>Pseudomonas aeruginosa</jats:italic> quorum sensing molecule N‐(3‐oxododecanoyl)‐<jats:sc>L</jats:sc>‐homoserine lactone (OdDHL) has been shown to suppress cytokine production in macrophages. We have examined the effect of OdDHL and related compounds on constrictor tone of porcine blood vessels. OdDHL (1–30 μ<jats:sc>M</jats:sc>) caused a concentration‐dependent inhibition of U46619‐induced contractions of the coronary artery through a largely endothelium‐independent mechanism, but was markedly less effective in the pulmonary artery. Quantitively similar effects to those produced by OdDHL were observed with N‐(3‐oxododecanoyl)‐<jats:sc>L</jats:sc>‐homocysteine thiolactone, a thiolactone derivative, while N‐3‐oxododecanamide, a lactone‐free acyl analogue, possessed 1/3rd the potency as a vasorelaxant. Neither N‐butanoyl‐<jats:sc>L</jats:sc>‐homoserine lactone nor <jats:sc>L</jats:sc>‐homoserine lactone (up to 30 μ<jats:sc>M</jats:sc>) were active. Our findings indicate that OdDHL inhibits vasoconstrictor tone of both pulmonary and coronary blood vessels from the pig. The vasorelaxant action of OdDHL appears to be primarily determined by the N‐acyl chain length, with a minor contribution by the homoserine lactone moiety.<jats:p><jats:italic>British Journal of Pharmacology</jats:italic> (1999) <jats:bold>128</jats:bold>, 845–848; doi:<jats:ext-link xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" xlink:href="10.1038/sj.bjp.0702870">10.1038/sj.bjp.0702870</jats:ext-link>