Abstract
Three young Thoroughbred racehorses with atrial fibrillation which had occurred during a race were treated by oral administration with quinidine sulfate. In all of them treatment was successful with a small total dose of the drug, either 10 or 25g. Blood quinidine levels at the time of defibrillation were about 1mg/l or below. After cardioversion, all the horses resumed racing with good success. Electrocardiographical data and some clinical findings during the treatment were described. Discussion was made briefly on the characteristics of atrial fibrillation in the racehorse.
