Adsorbed films of whey protein isolate (WPI), sodium caseinate, and pure beta-lactoglobulin (beta-L) at the air-water interface have been subjected to large and quite rapid area changes, in a Langmuir trough. Typically, the area was increased by a factor of 3 in 0.8 s. The resulting response of the surface tension (gamma) consisted of a rapid rise in gamma, followed by a much slower decay in gamma after cessation of expansion. The rise in gamma was reasonably well fitted by a simple Maxwell fluid model to give corresponding elastic (G) and viscous (eta) components. In general, G was dominant over eta, and G was higher for beta-L and WPI than for caseinate. G tended to be higher for whey protein films that had been aged, or at higher bulk protein concentrations, despite the fact that gamma for these systems was essentially the same before expansion. Also, for whey protein films that had been aged for prolonged periods, notably for pure beta-L, the decay in gamma was markedly slowed. It is proposed that these aging effects are due to aggregation, cross-linking, and possibly multilayer formation within the films that somehow prevent redistribution of the protein already within the film or inhibit the adsorption of protein from the bulk to the interface. Brewster angle microscopy of the films supports this proposition: WPI in particular formed persistent, apparently aggregated structures. This appears to limit the capacity of the whey proteins to stabilize foam bubbles under similar conditions of large scale surface deformation.