['Humans are more similar in transmural Purkinje and cardiac ion channel distributions to dogs than pigs. The Purkinje network in pigs is transmural but confined to the endocardium in dogs. Little is known about intramural activation during long-duration ventricular fibrillation (LDVF) given these differences. We tested the hypothesis that the transmural activation sequence is similar in sinus rhythm (SR) and LDVF in dogs as well as pigs, but different between species.', 'In six pigs and seven dogs, 50-60 plunge needles (six electrodes, 2-mm spacing) were placed throughout the left ventricle. Unipolar recordings were made for >10 minutes of LDVF. SR and LDVF activation times were grouped into waves by linking activations along each needle. Origin (earliest activation) and propagation direction were determined for each wave. The mean wave origin was significantly more endocardial in dogs than pigs for SR and 1 through 10 minutes of LDVF. Predominant propagation direction in LDVF and SR was endocardial to epicardial in dogs, but the opposite or equal in both directions in pigs. Fastest activation rate was epicardial in pigs, but endocardial in dogs with an increasing endocardial-to-epicardial activation rate gradient as LDVF progressed in dogs but not pigs.'] Is the transmural activation sequence in porcine and canine left ventricle markedly different during long-duration ventricular fibrillation?