(1) In chloralose anesthetized dogs, effects of the left and the right atrial stretch were studied in the same animal. Stretch of the sino-atrial region of the right atrium produced acceleration of the heart rate during, and reversal of response at the termination of, the stretch. Stretch of the left pulmonary vein-atrial junctional region evoked an initial decrease followed by an increase in heart rate. The responses were similar in all animals, despite initial heart rates ranging from 120 to 200 beats per minute. (2) Activity in vagal and sympathetic nerve branches innervating the heart was recorded simultaneously. Care was taken to identify the vagal fibers innervating the heart, and record their activity without contamination of sympathetic impulses. The right atrial stretch evoked an augmentation of sympathetic activity which reached its peak at 20 sec after the beginning of stimulus. The stimulus slightly increased the vagal activity; this change occurred slowly and reached its peak in about 40-60 sec after stretch. At the release of stretch, sympathetic activity generally showed a reversal of response, i.e. activity was inhibited for 30 sec. (3) Stretch of the left atrium produced biphasic changes in cardiac sympathetic nerves; their activity was strongly inhibited for the first 15 sec, then augmented throughout the remainder of the stretch. This effect lasted 30 sec after the cessation of stimulus. Effects on vagal cardiac nerve fibers were smaller; mild augmentation in activity was produced. The onset of this effect was faster than that seen in case of the right atrium stretch. (4) Reciprocal action between vagal and sympathetic cardiac nerves was obvious only in the early phase of left atrium stretch. Effects on the heart were determined by balances in activity of these antagonistic nerves. In contrast with what occurred in cardiac reflexes, carotid sinus distension even in the same animal produced a large increase in vagal activity, and near complete inhibition of sympathetic nerve activity. Thus, good reciprocal action between the two sets of nerves was demonstrated. A difference in the two types of reflexes was thus revealed. (5) Stretch of the right atrium evoked during carotid sinus distension caused an increase in heart rate from the new low level which was produced by baroreceptor activation. Vagal activity which was greatly augmented by sinus distension was decreased by atrial stretch, while previously inhibited sympathetic activity due to sinus distension was augmented by stretch of the atrium. The effect of stretch on vagal activity seems to depend to a degree on the prestimulus level. It is of interest that the powerful baroreceptor reflexes do not mask the cardiac reflexes studied. (6) The relative importance of sympathetic and vagal efferents to atrial stretch reflexes was discussed.