BACKGROUND: Cis-atracurium is more potent, and less likely to release histamine and has better autonomic and cardiovascular stability, compared to atracurium. It is eliminated mainly by Hofmann reaction and produces less amounts of laudanosine, the first metabolite which has the CNS stimulating property. However, drug interactions with several ions, such as magnesium that is popular in treating eclampsia have not yet to be known. METHODS: Using sciatic nerve-anterior tibialis preparation in the cat, we tried to find out whether cis-atracurium has the cumulation effect by administering four consecutive 100 microgram/kg of cis-atracurium. After confirming that the cis-atracurium had no cumulation effect, we proceeded the dose-response study to find out the ED50, ED95 and action duration of cis-atracurium in control group(II) and magnesium-pretreated group(III). After cumulative dose -response study, in the half of the group II and III, the infusion rate to maintain 90% twitch depression at least for 20 minutes was obtained, and afterwards, the recovery index was measured after the discontinuation of infusion. In the other half of the group II and III, while maintaining 90% twitch depression at least for 20 minutes through continuous infusion of cis-atracurium, we administered the 10 microgram/kg of neostigmine to measure the onset time, action duration and antagonism effect of the drug. RESULTS: The cis-atracurium did not have the cumulation effect and in group III, ED50 and ED95 were decreased significantly from 234+/-82 microgram/kg to 103+/-18 microgram/kg and from 399+/-123 microgram/kg to 173+/-28 microgram/kg, respectively, compared with group II. The action duration of cis-atracurium ED95 in group III(190+/-52 sec) was significantly longer than in group II(143+/-29 sec). The infusion rate of cis-atracurium to maintain 90% twitch depression was significantly higher in group II in the spontaneous recoveryand the neostigmine-induced antagonism(6.63+/-2.37 and 4.47+/-1.58 microgram/kg/min), respectively. CONCLUSION: The cis-atracurium do not have the cumulation effect. And, the magnesium potentiates the neuromuscular blocking actions of cis-atracurium. However, it has no effect on the antagonism action of neostigmine.