The aim of the present study was to estimate heterogeneity of ventricular repolarization in feline hearts in situ. Eighty-eight unipolar electrograms were recorded from subepicardial, midmyocardial, and subendocardial sites of the heart ventricles using plunge needle electrodes in open-chest anesthetized cats under sinus rhythm. Repolarization times (RTs) and activation- recovery intervals (ARIs) were determined for each lead and corrected to heart rate. The data are expressed in ms as Me (25%; 75%). ARIs were 279 (206; 284) in the subepicardium and 280 (230; 308) in the subendocardium in the base of the right ventricle, 206 (191; 261) in the subepicardium and 231 (223; 266) in the subendocardium in the apex of the right ventricle, 229 (213; 238) in the subepicardium and 256 (246; 280) in the subendocardium in the base of left ventricle, and 244 (209; 258) in the subepicardium and 268 (229; 281) in the subendocardium in the apex of the left ventricle. ARIs and RTs in the subendocardium were greater than those in the subepicardium (P<0.05). Transmural gradients of RTs and ARIs were found in the base and apex both of the right and left ventricle. ARIs increased from the apex to the base in the right ventricle [225 (206; 284) ^vs. 279 (221; 305), P<0.05]. No difference in ARIs between the base and apex of the left ventricle was observed [241 (222; 266) vs. 253 (216; 273), respectively]. Thus, these data suggest that there are the transmural and apicobasal repolarization gradients in feline hearts in situ. The study was supported by the Ural Branch of the Russian Academy of Sciences (project No. 12- C-4-1009).