Kallikrein-kininogen-kinin systems are now topics of widespread interest. The long-standing appreciation of their diverse pharmacological properties and biochemical characteristics is being supplemented by modern definitions of their cellular receptors' signal-transduction mechanisms and physiological and pathological roles. The assignment of important homeostatic responsibilities for kinins, including those in autocrine and paracrine signaling for skeletal and cardiac muscle energy metabolism, is now subject to definitive experimental evaluation via the availability of better kallikrein inhibitors, specific kinin receptor antagonists, and techniques of genetic manipulation.