Background Lidocaine and ropivacaine are often combined in clinical practice to obtain a rapid onset and a prolonged duration of action. However, the systemic toxicity of their mixture at different concentrations is unclear. This study aimed to compare the systemic toxicity of the mixture of ropivacaine and lidocaine at different concentrations when administered intravenously in rats.Methods Forty-eight male Wistar rats were randomly divided into 4 groups (n=12 each): 0.5% ropivacaine (group Ⅰ); 1.0% ropivacaine and 1.0% lidocaine mixture (group Ⅱ); 1.0% ropivacaine and 2.0% lidocaine mixture (group Ⅲ); and 1.0% lidocaine (group IV). Local anesthetics were infused at a constant rate until cardiac arrest. Electrocardiogram, electroencephalogram and arterial blood pressure were continuously monitored. The onset of toxic manifestations (seizure, dysrhythmia, and cardiac arrest) was recorded, and then the doses of local anesthetics were calculated. Arterial blood samples were drawn for the determination of local anesthetics concentrations by high-performance liquid chromatography.Results The onset of dysrhythmia was later significantly in group IV than in group Ⅰ, group Ⅱ, and group Ⅲ (P 0.05). The onset of seizure, cardiac arrest in group Ⅰ ((9.2±1.0) min, (37.0±3.0) min) was similar to that in group Ⅱ ((9.1±0.9) min, (35.0±4.0) min) (P>0.05), but both were later in group Ⅲ ((7.5±0.7) min, (28.0±3.0) min) (P