BACKGROUND: Lidocaine is useful as a local anesthetic and antiarrhythmic agent perioperatively. It may augment a neuromuscular block from both nondepolarizing and depolarizing muscle relaxants. Cisatracurium is a new muscle relaxant as an isomer of atracurium. We investigated the interaction of cisatracurium and lidocaine on the dose-response curve in vitro. METHODS: Institutional approvement was obtained. Forty male Sprague-Dawley rats (150 - 200 gm) were divided into four groups (control, lidocaine 0.01, 0.1, or 1ng/ml). The animals were anesthetized with 40 mg/kg pentobarbital. The hemidiaphragm with the phrenic nerve was dissected and mounted within 5 minutes in a bath containing 100 ml Kreb's solution at 32degreesC. The phrenic nerve was stimulated at supramaximal intensity by a Grass(R) 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 was added to the solution to obtain an initial concentration of 50ng/ml with saline 1 ml or lidocaine 0.01, 0.1 or 1ng/ml. When a stable 3 - 5 twitch inhibition was obtained after the first dose, additional cisatracurium was added to the Kreb's solution in increments of 25ng/ml to more than a 90% neuromuscular block. The data was analyzed by repeated measures of ANOVA. RESULTS: There was a significant decrease in the effective dose of cisatracurium needed to depress the twitch response in lidocaine 0.1ng/ml and 1ng/ml groups compared with the control group and with the lidocaine 0.01ng/ml group. CONCLUSIONS: We concluded that lidocaine will increase the sensitivity to cisatracurium in the hemidiaphragm preparation of rats.