BACKGROUND: Lidocaine is a useful local anesthetic and antiarrhythmic agent perioperatively. It may augment a neuromuscular block from both nondepolarizing and depolarizing muscle relaxants. Cisatracurium of benzylisoquinoline derivatives and rocuronium of aminosteroid derivatives are new nondepolarizing muscle ralaxants. We investigated interactions of cisatracurium or rocuronium with lidocaine in vitro. METHODS: Institutional approvement was obtained. Eighty male Sprague Dawley rats (150 - 200 g) were divided into 8 groups (control, lidocaine 1, 10 and 100microgram of both cisatracurium and rocuronium groups). The animals were anesthetized with phentobarbital 40 mg/kg I.P.. The hemidiaphragm with the phrenic nerve was dissected and mounted within 5 minutes in a bath containing 100 ml Kreb's solution at room temperature. The phrenic nerve was stimulated at supramaximal intensity by a Grass S88 stimulator through an SIU5 isolation unit. A twitch height was measured by a precalibrated Grass FT88 force displacement transducer and recorded with a Grass 79 polygraph. After stabilization of the twitch response, cisatracurium (50microgram), and rocuronium (100microgram) were added to the Kreb's solutions. After 10 minutes, we measured twitch height, and saline 1 ml or lidocaine 1, 10 or 100microgram was added to the Kreb's solution. After 10 minutes, we again measured twitch responses. The data was analyzed by repeated measures of ANOVA. RESULTS: There were significant depressions in the twitch response of cisatracurium in lidocaine 10microgram and 100microgram groups compared with the control group and lidocaine 1microgram group. In addition, the lidocaine 100microgram group was significantly depressed compared with the lidocaine 10microgram group. There were no significant depressions of the twitch response of rocuronium in lidocaine 1microgram, 10microgram, and 100microgram groups compared with the control group. CONCLUSIONS: We concluded that lidocaine will increase the sensitivity to cisatracurium in the hemdiaphragm preparation of rats. Lidocaine will be able to cause recurarization. There was not a statistically significant change in the rocuronium group but it was clinically significant.