Bradykinin is a peptide consisting of nine amino acids. It is a member of the kinin family, a class of molecules sometimes considered to be locally acting hormones. Bradykinin acts through cell surface receptors to elicit a series of biological responses, many of which have been well characterized at the whole organ or body level. However, little is known about the bradykinin receptor itself or its mechanisms of signal transduction, its function and its tissue distribution. Increasing evidence suggests that bradykinin is a member of a group of locally produced peptides which may act in a paracrine fashion as microenvironmental modulators of cell proliferation. Evidence for this derives from studies of the interaction between bradykinin and its receptor, receptor-effector coupling systems and in vitro studies of the biological effects of bradykinin. These areas, together with questions concerning the nature and number of different types of bradykinin receptors, form the main bulk of current interest in bradykinin research and are the subject of this review. The ability of bradykinin to synergize with other growth regulating ligands will also be considered.