This review article by William L. Smith provides an overview of eicosanoids, a group of compounds derived from polyunsaturated fatty acids, and their biochemical mechanisms of action. Eicosanoids, including prostaglandins, leukotrienes, and related hydroxy fatty acids, are synthesized from arachidonic acid through various pathways. The article discusses the pathways involved in the biosynthesis of these compounds, such as the cyclooxygenase and lipoxygenase pathways, and highlights the role of specific enzymes in each pathway. It also explores the mechanisms by which eicosanoids exert their biological effects, emphasizing the importance of G protein-linked receptors. The review explains how eicosanoids function as local hormones, coordinating the responses to other hormones and regulating cellular processes. Additionally, it delves into the physiological effects of前列腺素E2 (PGE2) in the kidney, particularly its role in water reabsorption and its interaction with AVP (antidiuretic hormone). The article concludes by suggesting that all eicosanoid actions can be understood as initiated by the interaction of eicosanoids with receptors coupled to G proteins, leading to changes in second messengers or intracellular ions.This review article by William L. Smith provides an overview of eicosanoids, a group of compounds derived from polyunsaturated fatty acids, and their biochemical mechanisms of action. Eicosanoids, including prostaglandins, leukotrienes, and related hydroxy fatty acids, are synthesized from arachidonic acid through various pathways. The article discusses the pathways involved in the biosynthesis of these compounds, such as the cyclooxygenase and lipoxygenase pathways, and highlights the role of specific enzymes in each pathway. It also explores the mechanisms by which eicosanoids exert their biological effects, emphasizing the importance of G protein-linked receptors. The review explains how eicosanoids function as local hormones, coordinating the responses to other hormones and regulating cellular processes. Additionally, it delves into the physiological effects of前列腺素E2 (PGE2) in the kidney, particularly its role in water reabsorption and its interaction with AVP (antidiuretic hormone). The article concludes by suggesting that all eicosanoid actions can be understood as initiated by the interaction of eicosanoids with receptors coupled to G proteins, leading to changes in second messengers or intracellular ions.