The peptide molecule apelin is used as a messenger in the cardiovascular system, brain, and in response to blood glucose. Its involvement has been specifically implicated in cancer progression and atherosclerosis. Apelin functions by interacting with a specific receptor protein. This receptor is located in membranes of many cells in the body. As a whole, proteins found in the membranes of cells such as heart muscle cells and neurons have important roles in controlling proper cellular and body function and are frequently used as drug targets. The activation of the apelin receptor, which occurs upon apelin binding, can cause dramatic physiological effects. These include changes in blood pressure and heart muscle contraction, changes to levels of blood insulin or other circulating hormones, and changes to behavior and neurological function. The HIV virus also makes use of the apelin receptor when it invades human cells, particularly in the brain and nervous system. The molecular-level basis of the interaction between apelin and its receptor is not understood. Using the technique of nuclear magnetic resonance spectroscopy (NMR), which allows us to locate and study individual atoms in a molecule, the atomic arrangement and motion in both apelin and its membrane-bound receptor will be studied. These NMR studies will be correlated to information obtained using other biophysical methods. By developing this type of atomic-level understanding, specific drugs can be designed to target either apelin or its receptor, increasing therapeutic treatment options for HIV infection, cancer and a variety of cardiovascular, central nervous system and metabolic disease conditions.