Chinese hamster ovary (CHO) cells were reversibly permeabilized by submitting them to short, high-intensity, square wave pulses (1.8 kV/cm, 100 microseconds). The cells remained in a permeable state without loss of viability for several hours at 4 degrees C. A new anisotropic peak with respect to control cells was observed on 31 P NMR spectroscopic analysis of the phospholipid components. This peak is only present when the cells are permeable, and normal anisotropy is recovered after resealing. Taking into account the fusogenicity of electropermeabilized cells, comparative studies were performed on 5% poly(ethylene glycol) treated cells. The 31P NMR spectra of the phospholipids displayed the same anisotropic peak as in the case of the electropermeabilized cells. In the two cases, this anisotropic peak was located downfield from the main peak associated to the phospholipids when organized in bilayers. The localization of this anisotropic peak is very different from the one of a hexagonal phase. We proposed a reorganization of the polar head group region leading to a weakening of the hydration layer to account for these observations. This was also thought to explain the electric field induced fusogenicity of these cells.