The review discusses how the level of free arachidonic acid is controlled in mammalian cells. Arachidonic acid is a precursor for eicosanoids, which are signaling molecules. The availability of arachidonic acid is limited by its esterified form in complex lipids, and its release depends on lipases. The three main questions addressed are: which lipases are involved, how they are controlled, and which substrates are hydrolyzed. Phospholipids, particularly glycerophospholipids, are the main source of arachidonic acid in mammalian cells, with most arachidonate esterified in the 2-acyl position. The release of arachidonic acid involves hydrolysis by phospholipases, and its re-esterification by acyltransferases. The balance between these processes determines the level of free arachidonic acid in the cell. Acyltransferases play a key role in controlling arachidonic acid levels by transferring it from arachidonoyl-CoA to phospholipids. The activity of these enzymes varies between tissues and is influenced by factors such as cyclic AMP and calcium. The low level of free arachidonic acid in the cytosol and plasma is likely due to the high affinity of lysophosphoglyceride acyltransferase for arachidonoyl-CoA. Phospholipases A1 and A2 are the main enzymes responsible for the release of arachidonic acid from phospholipids. The specificity of these enzymes for arachidonic acid is a topic of debate, with some studies suggesting a preference for arachidonic acid, while others indicate that the release is not specific. The subcellular location of phospholipases and their activity can vary between tissues, and the release of arachidonic acid may be influenced by factors such as the presence of other fatty acids and the activity of acyltransferases. The control of phospholipase activity is influenced by various factors, including calcium, cyclic AMP, and membrane structure. The role of calcium in controlling phospholipase activity is complex, and the effects of calcium ionophores such as A23187 on phospholipase activity are not always straightforward. Corticosteroids can affect arachidonic acid availability by influencing the enzymes involved in its synthesis and metabolism. They can also inhibit phospholipase activity and arachidonic acid release, suggesting that they may act through their cytosolic receptors. The release of arachidonic acid from phosphatidylinositol is a complex process involving the action of phosphodiesterase (phospholipase C) and the recycling of diacylglycerol. The turnover of phosphatidylinositol is important for maintaining the balance of arachidonicThe review discusses how the level of free arachidonic acid is controlled in mammalian cells. Arachidonic acid is a precursor for eicosanoids, which are signaling molecules. The availability of arachidonic acid is limited by its esterified form in complex lipids, and its release depends on lipases. The three main questions addressed are: which lipases are involved, how they are controlled, and which substrates are hydrolyzed. Phospholipids, particularly glycerophospholipids, are the main source of arachidonic acid in mammalian cells, with most arachidonate esterified in the 2-acyl position. The release of arachidonic acid involves hydrolysis by phospholipases, and its re-esterification by acyltransferases. The balance between these processes determines the level of free arachidonic acid in the cell. Acyltransferases play a key role in controlling arachidonic acid levels by transferring it from arachidonoyl-CoA to phospholipids. The activity of these enzymes varies between tissues and is influenced by factors such as cyclic AMP and calcium. The low level of free arachidonic acid in the cytosol and plasma is likely due to the high affinity of lysophosphoglyceride acyltransferase for arachidonoyl-CoA. Phospholipases A1 and A2 are the main enzymes responsible for the release of arachidonic acid from phospholipids. The specificity of these enzymes for arachidonic acid is a topic of debate, with some studies suggesting a preference for arachidonic acid, while others indicate that the release is not specific. The subcellular location of phospholipases and their activity can vary between tissues, and the release of arachidonic acid may be influenced by factors such as the presence of other fatty acids and the activity of acyltransferases. The control of phospholipase activity is influenced by various factors, including calcium, cyclic AMP, and membrane structure. The role of calcium in controlling phospholipase activity is complex, and the effects of calcium ionophores such as A23187 on phospholipase activity are not always straightforward. Corticosteroids can affect arachidonic acid availability by influencing the enzymes involved in its synthesis and metabolism. They can also inhibit phospholipase activity and arachidonic acid release, suggesting that they may act through their cytosolic receptors. The release of arachidonic acid from phosphatidylinositol is a complex process involving the action of phosphodiesterase (phospholipase C) and the recycling of diacylglycerol. The turnover of phosphatidylinositol is important for maintaining the balance of arachidonic