<jats:title>Abstract</jats:title><jats:p>The pharmacological effects of laurotetanine on rat isolated thoracic aorta were examined. The contraction of aortic rings caused by high potassium (60 Mm) and cumulative concentrations of calcium (0†03–3 Mm) was inhibited by 3–50 μm laurotetanine in a dose-dependent manner with an IC50 value of 19·8 ± 3·6 μm (n = 6) in a 1 Mm Ca2+ medium. The phenylephrine (3 μm)-induced contraction was also inhibited by laurotetanine. Its effect was more marked on the tonic contraction than on the phasic contraction and was not easily washed-out. On addition of laurotetanine during the tonic contraction, relaxation could also be observed. This relaxing effect was not antagonized by indomethacin (20 μm) and was still seen in denuded aorta or in the presence of nifedipine (1 μm). The caffeine (20 Mm)-induced contraction was not affected by laurotetanine. cAMP and cGMP levels of aorta were not changed by laurotetanine. It is concluded that laurotetanine relaxed the rat thoracic aorta mainly by suppressing the Ca2+ influx through both voltage- and receptor-operated calcium channels.