The interaction of tetracaine and lidocaine hydrocholorides with both zwitterionic and charged phospholipid species was investigated by the monolayer technique. A general film expansion on interaction with anesthetics is seen, accompanied by an increase in the surface potential and surface dipole moment. At higher anesthetic concentrations, however, the surface potential decreases and in the case of zwitterionic phosphatidylcholine and phosphatidylethanolamine species becomes even less than normal. There are differences between the effects of lidocaine and tetracaine, The latter is more potent in inducing film expansion, and at higher concentration seems to exert a hydrolyzing effect on the zwitterionic phospholipids. The results suggest that local anesthetics may act by two different mechanisms, and via enhancement of membrane capacitance.