Objective  To test electrophysiological heterogenity and different response to class Ⅲ agents of different regions in the guinea pig left ventricle.Method  The study used a dermatome to obtain thin strips of free wall of left ventricle in guinea pig and standard microeletrode techniques to record transmembrane activity.Results  Action potential duration measured at 90% repolarization (APD    90) was significantly longer in sub-endocardium cells than in mid-myocardium or sub-epicardium cells at all basic cycle lengths (BCLs) tested.At a BCL of 0.2Hz,APD    90 was (233±3),(216±2) and (210±3)ms in sub-endocardium,mid-myocardium,and sub-epicardium respectively;while at a BCL of 3.3Hz,it was (172±4),(  163±3) and (156±3)ms respectively.Thus APD-rate relations was more pronounced in the sub-endocardium.Sotalol-induced prolongated APD    90 was similar in the three regions studied.Conclusions  There are significant transmural gradients in the electrophysiological properties of sub-epicardium,mid-myocardium, and sub-epicardium from the free wall of guinea pig left ventricle.Sub-endocardial myocytes have a longer APD    90 and the pronounced rate dependence in contrast to mid-myocardium and sub-epicardiac myocytes sotalol can not reduce transmural gradients.