The cytochrome P-450 pathway of AA metabolism is widely distributed and gives rise to a diversity of products affecting basic biological mechanisms such as vascular reactivity and transport function in critical nephron segments. P-450-AA metabolites may participate in receptor-mediated signal transduction and may act as second messengers. The synthesis of P-450-AA products can be altered by pharmacologic probes and is affected by pathophysiological conditions. That this review has centered on the circulation and renal function should not be interpreted as minimizing the importance of P-450-AA metabolites in other organs and systems for, most assuredly, they will be shown to constitute an essential component of organ function in sites other than those addressed here.