The ability of sodium dodecyl sulfate (SDS) to traverse phosphatidylcholine bilayers was examined by fluorescence spectroscopy. To this end, we measured the interaction of the anionic fluorescent probe 2-(p-toluidinyl)naphthalene-6-sodium sulfonate present in the outer vesicle leaflet with the SDS monomers incorporated in this structure. The surfactant transbilayer movement, or "flip-flop", was measured from the fluorescence intensity changes due to the interaction of the liposome/probe with SDS versus incubation time. This effect was quantified as the resulting variations in the surface potential (psi(o)) of liposomes. When the SDS concentration increased, psi(o) rose due to the electronegative contribution of the sulfate group incorporated in the bilayer surface. Increased periods of incubation resulted in a decreased psi(o) and, consequently, in a fall in the number of surfactant molecules in the outer vesicle leaflet. This variation was associated to the SDS "flip-flop". The maximum "flip-flop" (of about 50%) was always detected at a very low surfactant concentration, and the effective molar ratio of surfactant to PC for this maximum was always a constant value (0.02 mol/mol). Although the incorporation of SDS monomers to the bilayer surface was a very rapid process, the "flip-flop" rate of these monomers across lipid bilayer was very slow and time dependent with an enhanced kinetics between 10 and 90 min after mixing.