A kinin is any of various structurally related peptides, such as bradykinin and kallidin. They are members of the autacoid family. Kinins are peptides that are cleaved from kininogens by the process of kallikreins. Kallikreins activate kinins when stimulated.

It is a component of the kinin-kallikrein system.

Their precursors are kininogens. Kininogens contain a 9-11 amino acid bradykinin sequence.

In botany, the plant hormones known as cytokinins were first called kinins; the name was changed to avoid confusion.

Kinins are short-lived peptides that cause pain sensation, arteriolar dilation, increase vascular permeability, and cause contractions in smooth muscle. Kinins transmit their effects through G protein-coupled receptors.

Kinins act on axons to block nerve impulses, which leads to distal muscle relaxation. They are also potent nerve stimulators which are mostly responsible for the pain sensation (and sometimes itching). Kinins increase vascular permeability by acting on vascular endothelial cells to cause cell contraction. Concomitantly, they induce local expression of adhesive molecules. Together they increase white blood cell adhesion and extravasation. Kinins are rapidly inactivated by the proteases generated locally during the aforementioned processes.

They act locally to induce vasodilation and contraction of smooth muscle. Kinins function as mediators for inflammatory responses by triggering the immune system. They are also able to regulate cardiovascular and renal (kidney) function by mediating the effects of ACE inhibitors. Reduced kinin activity can result in high blood pressure, sodium retention, and the narrowing of blood vessels.

Aspirin inhibits the activation of kallenogen by interfering with the formation of the kallikrein enzyme, which is essential in the process of activation.