BACKGROUND: The cardiovascular effects of muscle relaxants may be produced by a muscarinic receptor block, ganglion block, increased noradrenaline release and blockage of it's reuptake or histamine liberation. However certain analogues of steroidal muscle relaxants directly cause relaxation of isolated vascular smooth muscles. Rocuronium is the 2-morpholino, 3-desacetyl, 16-N-allyl-pyrrolidino derivative of vecuronium, known to have a relative lack of ganglion blocking, sympathomimetic effects or histamine release. The aim of this study was to investigate the effect and action mechanism of rocuronium on isolated aortic smooth muscle in rats. METHODS: The ability of rocuronium to elicit a direct relaxant effect on vascular smooth muscle has been studied using isolated rat thoracic aortic rings contracted with phenylephrine (PE). Each ring of aortic arteries was suspended on wire supports in a 20 ml tissue bath under 2 g of resting tension. All tissues were bathed in Tris Tyrode solution at 37degrees C and 100% oxygen was supplied. RESULTS: Rocuronium 10(-4) M and 10(-2) M shifted the cumulative concentration-effect curves of PE to the right significantly (P<0.05). This relaxation effect of the aortic rings was not reversed with L-NAME or methylene blue pretreatment. However indomethacine shifted this curve to the right, and intracellular calcium release and extracellular calcium influx decreased significantly (P<0.05). CONCLUSIONS: From the results obtained with this experiment, it is concluded that the relaxation effects of rocuronium is not endothelium dependent and in part, is related with cyclooxygenase inhibition. Rocuronium inhibited intracelluar calcium release and extracelluar calcium influx.