Atropine/dobutamine stress echocardiography has recently been developed for detection and prognosis of heart diseases in equine patients. This pharmacological stress test is known to increase cardiac output by increasing heart rate and load dependant measures of myocardial contractility, and to decrease preload. This study was performed to evaluate the effects of this stress test on left ventricular afterload. Six healthy adult horses were studied. The stress test consisted in an intravenous injection of atropine (35mg/kg) followed by 4 incremental steps from 2 to 8 mg/kg/min of dobutamine infusion. Mean arterial pressure (MAP) was measured via a catheter inserted into the transverse facial artery and cardiac output (CO) was measured by thermodilution. Systemic vascular resistance (SVR) was calculated from MAP and CO and left ventricular wall stress (WS) from MAP and left ventricular dimensions obtained by M-mode echocardiography. All values were compared at baseline and at each step of pharmacological stimulation by regression analysis. SVR decreased from 323.9 31.2 at baseline to 136.7 36.1 dynes * sec *cm^-5 at maximal stimulation while WS ranged from 61.3 7.6 at baseline to 50.9 8.4 g/cm² during pharmacological stimulation. Afterload is the force against which the ventricle must act in order to eject blood. It is very difficult to measure in clinical settings and only rough estimation of afterload can be performed in intact animals. These results suggest that at least some elements of left ventricular afterload are decreased in horses submitted to the newly developed atropine/dobutamine stress echocardiography test. A similar decrease in afterload has been described in men submitted to dobutamine stress echocardiography.