Small intracellular mediators relay signals from cell surface receptors to various intracellular targets, amplifying the cellular responses to extracellular signals. They do this by binding to and influencing the behavior of target proteins. Examples of small intracellular mediators include: Ca2+, cyclic AMP, and diacylglycerol.

• Cyclic AMP activates protein kinase A (PKA), which then phosphorylates target proteins. 
• Diaglycerol contributes to the formation of glycerophospholipids and functions as lipid-based second messengers in cellular signaling. 
• Calcium ions are involved in numerous cellular processes, including muscle contraction, cell signaling and neurotransmitter release.