The role played by the cardiac sympathetic nerves and arterial baroreceptors in the cardiovascular responses to acute volume loading was studied in conscious dogs. In 15 normally innervated animals, mean arterial pressure rose 10 mmHg, heart rate increased 38 beats/min and cardiac output 1,696 ml/min, while peripheral resistance decreased 0.99 PRU. Neither bilateral baroreceptor denervation, dorsal root sections (T1-T5) or surgical interruption of the left ansa subclavia altered the above responses to acute volume loading. Bilateral section of the ansa subclavia (total cardiac sympathectomy) significantly reduced the heart rate response from 35 +/- 5 to 20 +/- 5 beats/min but did not alter other changes. A similar reduction in heart rate response was observed following selective section of the right ansa subclavia. Intravenous infusions of epinephrine augmented the heart rate response in both normally innervated and cardiac sympathectomized dogs. It is suggested that although the primary efferent pathway for the reflex trachycardia is via the vagus, responses are modulated by sympathetic neural activity. Additionally, the nervous system was not shown to play a measureable role in the observed peripheral resistance changes.