Objective To evaluate the effects of bupivacaine and ropivacaine on guinea pig papillary muscle action potential Methods The guinea pig papillary muscle preparation was exposed to ropivacaine (1, 3 or 5?g?ml -1 ) or bupivacaine ( 1, 3 and 5?g?ml -1 ) respectively, to measure maximum rate of depolarization (Vmax ), action potential amplitude (APA),action potential duration at 50% of repolarization(APD 50 ), and action potential duration at 90% of repolarization(APD 90 ) Results Vmax significantly decreased after exposure to both agents at all concentrations, Vmax decreased by 27 4% with ropivacaine at 5?g?ml -1 , and the degree was similar to that with bupivacaine at 3 ?g?ml -1 (28 4%) ,but was significantly lower than that with bupivacaine at 5 ?g?ml -1 (42 6%) (P