Local regulators secreted by cells that make them convey messages between neighboring cells a process referred to as paracrine signaling. Local regulators act on nearby target cells within seconds or even milliseconds, eliciting cell responses more quickly than hormones can. Some local regulators have cell–surface receptors; others have intracellular receptors. Binding of local regulators to their specific receptors triggers events within target cells similar to those elicited by hormones.

Several types of chemical compounds function as local regulators. Many neurotransmitters, the key local regulators in the nervous system, are amino acid derivatives. Among peptide/ protein local regulators are cytokines, which play a role in immune responses, and most growth factors, which stimulate cell proliferation and differentiation. Growth factors must be present in the extracellular environment in order for many types of cells to grow, divide and develop normally.

The nitric oxide (NO) gas is another example of a local regulator. When the blood oxygen level falls, endothelial cells in blood vessel walls synthesize and release NO. Nitric oxide activates an enzyme that relaxes the neighboring smooth muscle cells, which in turn dilates the vessels and improves blood flow to tissues. Nitric oxide has other functions as well: In the nervous system, it can function as a neurotransmitter and NO secreted by certain white blood cells can kill bacteria and cancer cells in body fluids.