The effect on the heart rate of stretching the superior vena cava 3-4 mm above its entrance to the right atrium was examined in rats anesthetized with chloralose. The superior vena cava was stretched 41 times in 16 rats. There was a mean increase of 24.1 +/- 1.9 (SE) beats/min. This response was abolished (6 rats) by sympathetic blockade (with propranolol hydrochloride and bretylium tosylate) and by bilateral vagosympathectomy (5 rats). Application of lidocaine to the cervical vagi also abolished the response in a reversible manner (4 rats). In five rats, histological examination of the superior vena cava revealed an abundance of nerves, which formed a net in the stretched region. Complex unencapsulated nerve endings similar to those demonstrated in the dog and cat were not found.