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 <0.01), but there was no significant difference in these groups (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<0.05). The onset of each toxic manifestation was significantly later in group IV than in group I (P<0.01). The plasma concentrations of the lidocaine-alone group at the onset of dysrhythmia (DYS), cardiac arrest (CA) ((41.2±6.8) min, (59.0±9.0) min) were higher than those of the ropivacaine alone group ((20.5±3.8) min, (38.0 ±8.0) min) (P <0.05). The plasma concentrations of ropivacaine inducing toxic manifestation were not significantly different among groups Ⅰ, Ⅱ, and Ⅲ (P>0.05).Conclusions The systemic toxicity of the mixture of 1.0% ropivacaine and 2.0% lidocaine is the greatest while that of 1.0% lidocaine is the least. However, the systemic toxicity of the mixture of 1.0% ropivacaine and 1.0% lidocaine is similar to that of 0.5% ropivacaine alone.