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Rapid atrial activation causes electrical remodeling that promotes the occurrence and maintenance of atrial fibrillation. The aim of this research was to compare the relationship between mechanical remodeling and atrial electrophysiology. Eight dogs (beagles) were subjected to rapid atrial pacing (AP) at 400 beats/min for 16 days. After a complete recovery of electrical variables and left ventricular function evaluated by echocardiography, they underwent high-rate ventricular pacing (VP) at 240 beats/min of equal duration. In half of them, the study was started by VP and in the other half by AP. Left atrial systolic function was assessed by transesophageal echocardiography. Atrial effective refractory period (AERP) at a basic cycle length of 400 ms decreased significantly after either type of pacing (AP: 115 +/- 17 ms, VP: 136 +/- 22 ms) compared with baseline values (153 +/- 23 ms); the difference between tachycardias was significant too (p < 0.02). Significant increases (p < 0.05) in left atrial dimensions (LA-A) (AP: 2.41 +/- 0.23 cm ,VP: 2. 43 +/- 0. 34 cm vs. basal: 2. 16 +/- 0. 21 cm) indicated atrial dilatation after either type of pacing, the differences between two groups being insignificant. Atrial reversal pulmonary venous flow (AR velocity) decreased in AP (-0.13 +/- 0. 02 m/s) and VP (-0. 17 +/- 0. 04 m/s). The difference was highly significant as compared to basal values (-0.25 +/- 0.05 m/s) and also with respect to both tachycardias (p < 0.01). In both groups, atrial remodeling occurred in a relatively short period of time. The echocardiographic findings suggested that left atrial systolic function was significantly more disturbed in the AP group than in the VP group. Mechanical changes are an important substrate of electrical remodeling, yet the deterioration of electrical variables was more pronounced in AP than in VP.