The objective of this work was to further our understanding of the influence hemodynamic forces have on the cardiovascular system. Specifically, the effect of shear stress on platelet aggregability under two pathophysiological conditions was studied. Elevated shear stress levels in stenosed vessels induce platelet aggregation. Increased plasma catecholamine concentrations have also been implicated in the onset of acute coronary ischemic syndromes. This first study was designed to examine the synergistic platelet activation by the interaction of shear stress and epinephrine. Platelets suspensions sheared at subthreshold levels in a cone and plate viscometer showed little or no aggregation unless pretreated with subthreshold concentrations of epinephrine. Monoclonal antibody blockade of glycoproteins (Gp) Ib and IIb/IIIa showed that the synergistic platelet aggregation required functional Gp IIb/IIIa but could partially bypass Gp Ib-von Willebrand factor (vWF) interaction. Binding studies indicated that fibrinogen is required for platelet priming, but a functional role for vWF could not be ruled out. Synergistic platelet aggregation was also observed in whole blood following exercise to elevate endogenous levels of catecholamines in blood donors. Vascular smooth muscle cells (SMCs) are exposed to fluid dynamic shear stress at sites of vessel wall damage. This second study was designed to examine the effects of shear stress on the production of platelet inhibitory nitric oxide (NO) and carbon monoxide (CO) from SMCs. A modified cone and plate viscometer was used to subject monolayers of SMCs to arterial levels of shear stress. Shear stress was observed to induce mRNA and protein production of inducible heme oxygenase (HO-1), which generates CO, but not to affect inducible NO synthase, in a shear stress and time dependent manner. Washed platelets, used to detect the production of CO from SMCs, contained elevated levels of cGMP and demonstrated an inhibited aggregation response to ADP following coincubation with sheared SMCs. The elevated intra-platelet cGMP levels and inhibited aggregation response could only be blocked by incubating the SMCs with the HO-1 inhibitor tin protoporphyrin. These results indicate that shear stress may play a variety of roles with regard to platelet aggregability under pathological conditions, helping to control blood fluidity.