The actions of bee venom melittin and delta-lysin from Staphylococcus aureus on membranes have been monitored by solid-state deuterium and phosphorus NMR and shown to differ depending on temperature and on the lipid-to-peptide molar ratio Ri. In the gel phase of phosphatidylcholine model membranes, for lipid-to-peptide ratios Ri greater than 15, melittin induces isotropic lines interpreted as reflecting the presence of small discoidal structures, whereas delta-lysin does not. These small objects are metastable, that is, within a time-scale of hours they return to large lipid bilayers. The kinetics of this process depend on the lecithin chain length. In the fluid phases, at temperatures greater than that of the gel-to-fluid transition Tc, analysis of the quadruplar splittings in terms of chain ordering indicates that both melittin and delta-lysin similarly disorder the membrane. At temperatures above but close to Tc, melittin preferentially orders the center of the bilayer, while delta-lysin promotes ordering throughout the entire bilayer thickness. These effects are interpreted as reflecting different locations of the peptides with respect to the membrane surface. The addition of greater amounts of toxins, Ri = 4, on phosphatidylcholine model membranes induces very small structures irrespective of the temperature in the case of melittin, but only above Tc for delta-lysin. NMR spectral features similar to those characterizing the small fast-tumbling objects with phosphatidylcholine are also observed with egg phosphatidylethanolamine and erythrocyte membranes. The formation of small structures is thus inferred as a general process which reflects membrane supramolecular reorganization.