Twenty-four hour ambulatory electrocardiographic recording using a Medilog tape recorder was attempted in 16 healthy pet dogs weighing between 18.4 and 34 kg, while they were living in familiar surroundings. Full 24-hour records were obtained from 10 of them and recordings of more than 10 hours duration from two others; and the findings in these recordings were similar. Maximum heart rates ranged between 110 and 300 beats/minute and the minimum rates ranged between 17 and 46 beats/minute. Intermittent ventricular premature complexes were recorded from these apparently healthy dogs and all but one dog demonstrated a sinus pause, longer than two seconds; the longest pause was 5.7 seconds. The pauses were associated with marked sinus arrhythmia and occurred in both brachycephalic and non-brachycephalic breeds.