Previous investigators demonstrated that distension of the left atrium (LA) or pulmonary veins (PV) evokes reflex changes in heart rate (HR) and mean arterial blood pressure (MABP). This study was designed to determine whether stimulation of receptors in these areas could evoke reflex changes in left ventricular (LV) maximum contractility (Vmax) in anesthetized dogs. Balloons were placed in the left atrial appendage or in two right pulmonary veins to stimulate mechanoreceptors in these regions without directly altering other cardiorespiratory pressures. Distension of either the LA or PV increased LV contractility and decreased MABP. LA distension evoked a vagally mediated increase in HR when predistension HR was less than 140 beats/min but did not change HR when predistension HR was greater than 140 beats/min. Reflex changes in inotropic state and MABP were not altered by bilateral vagotomy or carotid sinus denervation. In dogs with stellate ganglia and sympathetic chains sectioned and vagi intact, PV or LA distension decreased LV contractility and MABP. Responses were eliminated when both vagal and sympathetic pathways were cut. We conclude that distension of the LA or the PV and LA junction evokes a sympathetically mediated increase in LV contractility.