Since the second messenger role was proposed for the products of inositol phospholipid hydrolysis, considerable progress has been made in our understanding of the biochemical mechanism of the intracellular signaling network. It is now becoming evident that stimulation of a cell surface receptor initiates a degradation cascade of various membrane lipid constituents. Many of their metabolites have potential to induce, intensify, and prolong the activation of protein kinase C that is needed for sustained cellular responses.