AVP is a nine amino acid peptide hormone that plays a primary role in the regulation of renal water excretion and a secondary role in the regulation of cardiovascular function in mammals. AVP acts on epithelial cells of the urinary tract to augment the reabsorption of water, resulting in concentration of urine and dilution of blood serum. It also stimulates the constriction of capillaries and arterioles, elevating blood pressure. Normal levels of AVP in serum are 0.4-5.2 pg/ml. Plasma levels of AVP are elevated in patients with major and also anxious-retarded depression, as well as in patients with congestive heart failure. Cayman's AVP EIA is a competitive assay that can be used for the measurement of AVP from plasma and serum.