The purpose of the present study was to identify the cardiac vagal nerve (CVN) of the turtle, to characterize its fiber composition, and to correlate this composition with cardioinhibitory functions. Turtles (Ocadia sinensis) were anesthetized with sodium pentobarbital. The CVN was identified anatomically as a thoracic vagal branch going to the heart. Transection or reversal block of this branch completely abolished the negative chronotropic and inotropic effects produced by ipsilateral cervical vagal stimulation. Electron microscopic examination of the CVN revealed that it is comprised of 500 to 1800 axon fibers. Among these, 86% were unmyelinated and 14% were myelinated fibers. Compound action potentials of the CVN consisted of A, B, and C groups. A decrease in the heart rate or a reduction of ventricular contractility was observed with electrical stimulation of the cervical vagus at an intensity which activates the B-fiber group. When the stimulus intensity increased to recruit both the B- and C-fiber groups, maximal cardioinhibitory effects were observed. The negative chronotropic effect of the right vagus was greater than that of the left vagus with low-frequency stimulation. In contrast, stimulation of the left vagus produced greater negative inotropic effect. These data indicate that the turtle heart is innervated by a single pair of CVN. The cardioinhibitory functions are subserved by small myelinated and large unmyelinated fibers. Functionally distinct vagal neurons may be distributed unevenly in the turtle brain, such that the right vagal nerve contains more chronotropic while the left more inotropic motor fibers.