Abstract: The invention relates to the use of a 5-HT4 receptor antagonist in the manufacture of a medicament for the prophylaxis or treatment of atrial remodelling in a mammal. Preferably, the antagonist is N-[(1-nbutyl-4-piperidinyl)methyl]-3,4-dihydro-2H-[1,3]oxazino[3,2-a]indole-10-carboxamide (SB 207266) or a pharmaceutically acceptable salt thereof. The invention also relates to the use of SB 207266 or a pharmaceutically acceptable salt thereof in the manufacture of a medicament for the treatment or prophylaxis of atrial fibrillation in a mammal by administering to the mammal a daily oral or parenteral dosage regimen of about 0.2 mg to 1.0 mg of the SB 207266 or salt thereof per kg of total body weight (measured as the free base). The invention also relates to the use of SB 207266 or a pharmaceutically acceptable salt thereof in the prophylaxis or treatment of atrial arrhythmia in a mammal by administration of the SB 207266 or salt thereof on the first day at a loading dose of about 1.2 to about 2.