High-density lipoprotein (HDL), apolipoprotein A-I (apoA-I), and the principal high-affinity HDL receptor, scavenger receptor class B type I (SR-BI), are antiatherogenic and beneficial to the response to vascular injury. However, the fundamental mechanisms underlying these properties remain complex and not well understood. Recent work in both cell culture and in mice indicates that HDL causes robust activation of endothelial nitric oxide synthase (eNOS), and that this effect is mediated in endothelial cell caveolae by SR-BI through a process that requires apoA-I binding. Further studies have revealed that HDL stimulates eNOS through src- and PI3 kinase-mediated signaling, which leads to parallel activation of Akt and mitogen-activated protein kinases and their resultant independent modulation of the enzyme. As such, signaling initiated by HDL increases the production of the potent atheroprotective molecule nitric oxide, and this novel mechanism of action may be critically involved in the impact of the lipoprotein on vascular health and disease.