The aim of this study was to determine the effects of chronic fluorosis on the electrocardiogram (ECG) in dogs. A total of sixteen mixed breed dogs (8 fluorotic and 8 healthy), 2 to 3 years old and weighing 18-30 kg, were used in this study. The eight dogs with chronic fluorosis were obtained from the Tendurek Mountain region (altitude about 2000 m) in Eastern Anatolia. All ECG waves were observed in all of the leads. Sinus bradycardia was seen in dogs with chronic fluorosis. Prolonged P-Q interval (P<0.05) was present. The duration and amplitude of T wave was higher and longer (P<0.05) in the fluorosis group. As a result, the number of heart beats/minute was decreased significantly (p<0.05) from 120.4+/-15 in the control group to 80.7+/-14 in dogs with fluorosis. The average electrical axis of the heart changes were between +60degrees and +100degrees in both groups. The average electrical axis was +80degrees in the control group and +75degrees in the fluorotic dogs.