Vasodilator drugs are widely used in the management of cardiovascular disease. They decrease systemic vascular resistance, but they also may influence vascular arterial compliance. This study evaluated the effects of three vasodilators - nitroprusside, nitroglycerin, and hydralazine - on vascular compliance using impedance parameters determined by pulse contour and Fourier analyses. The open chest study was performed on anesthetized dogs. Mean arterial pressure decreased by a minimum of 20% after vasodilator intervention. The decrease in systemic vascular resistance was significant (p < 0.01) only after hydralazine treatment. Proximal compliance increased after administration of all drugs, but the increase was not statistically significant. Distal compliance determined by pulse contour analysis increased by 60 to 120% after all three drug treatments (p < 0.05 for nitroprusside, p < 0.02 for nitroglycerin and hydralazine). Characteristic impedance from Fourier analysis responded variably, and changes were not statistically significant. The sensitivity of changes in distal compliance as a marker for the vascular effect of these drugs suggests that it might be used as a more reliable guide than blood pressure or vascular resistance in monitoring clinical response to such intervention. The more traditional measure of characteristic impedance provides a vascular measurement that is less sensitive than distal compliance to the effects of these vasodilator drugs.