Dynamic surface pressure (pi)-surface area (A) characteristics were measured using periodic changes in the surface area of an unsaturated phospholipid thin film formed at an air/water interface. Under appropriate conditions, the dynamic pi-A curve traces a single closed hysteresis loop after several cycles of compression and expansion. In the presence of local anesthetics, such as tetracaine, prilocaine, bupivacaine, mepivacaine, lidocaine, and procaine, the loop area of the dynamic pi-A curve decreases. Fourier transformation of the time-dependent change in the surface pressure is useful for analyzing the shape of the hysteresis loop, which reflects nonlinear viscoelastic properties, in a quantitative manner. It is clear that the most significant effect of local anesthetics is the reduction of the viscosity of the DOPC film at the interface. These experimental findings regarding dynamic pi-A characteristics are examined with the aid of a computer simulation by taking into account of the cooperative aggregation of phospholipid domains at an air/water interface.