The fusion of membrane bilayers is an ubiquitous cellular process. Exocytosis, organelle formation, cellular trafficking, cell division, fertilization, and numerous other cellular activities all involve membrane fusion. Fusion can be rapid, occurring on the order of seconds or minutes. In viral infection, specific glycoproteins mediate the fusion of the viral lipid envelope with a cellular membrane. Viral fusion glycoproteins typically contain a segment which embeds into the target cellular membrane, referred to as the fusion peptide. The mechanism of action of viral fusion proteins is still not certain; in particular, the extent and rate of insertion of the fusion peptide are not well quantified. In this report, we use micropipet aspiration and video microscopy of large unilamellar phosphatidylcholine vesicles to determine the membrane area expansion resulting from the insertion of fusion peptides into the lipid bilayer. The fusion peptide of the viral fusion protein, influenza hemagglutinin, inserts into phosphatidylcholine bilayers, resulting in an increase in the membrane area on a time scale (i.e., seconds) similar to that of viral fusion. Following peptide insertion, porous defects form in minutes. We show that chemical changes in the Nand C-termini of this peptide can either eliminate, decrease, or enhance surface activity of the peptide; particularly the propensity to form pores can be diminished. In control studies, the well-studied lyric peptide, melittin, similarly increases the membrane area and forms pores in the membrane. The observation that the wild-type influenza fusion peptide and melittin each form pores below the areal expansion limit of 5% suggest that membrane disrupting proteins act through specific and localized perturbation.