It has been shown that the arterial baroreflex is depressed in heart failure. The role of alterations in afferent discharge as a possible mechanism for this depression has not been investigated previously. Single unit aortic baroreceptor activity was recorded from six normal dogs and from nine dogs, each with a chronic aorto-caval fistula (AVF). At the time of the acute experiment, mean arterial blood pressure (MABP) was not significantly different in the two groups of dogs; however, pulse pressure was significantly higher in the AVF dogs (45.7 +/- 2.4 mm Hg vs, 24.4 +/- 2.0 mm Hg; p less than 0.001). Left ventricular end-diastolic pressure (LVEDP) was higher in the AVF dogs (31.3 +/- 2.0 vs 5.6 +/- 1.8 mm Hg; p less than 0.001). AVF dogs had elevated heart weight/body weight ratios. The relationship of systolic aortic pressure to systolic discharge was examined by changing aortic pressure with aortic and vena caval occluders. The peak gain (normalized to maximum discharge) averaged 2.19 +/- 0.27 in the normal dogs compared to 1.15 +/- 0.09 in the AVF group (p less than 0.01). Saturation pressures and maximum discharge rates were greater in the AVF dogs although the threshold pressures were not different in the two groups. This data suggests that there is an attenuated response of aortic baroreceptor discharge in dogs with chronic volume overload and this abnormality may partially be responsible for the abnormal baroreflex in heart failure.