This study describes a method for gene transfer into mouse L cells using electroporation in high electric fields. Electric impulses (8 kV/cm, 5 μs) significantly enhance DNA uptake into cells. When linear or circular plasmid DNA containing the herpes simplex thymidine kinase (TK) gene is added to a suspension of mouse L cells deficient in the TK gene and then exposed to electric fields, stable transformants are formed that survive in HAT selection medium. At 20°C, after three successive electric impulses followed by 10 minutes for DNA entry, 95 (±3) transformants per 10⁶ cells and per 1.2 μg DNA are obtained. This method is simple, easily applicable, and highly efficient compared to biochemical techniques. The mechanism of DNA transport through cell membranes is not fully understood, but a physical model, the 'electroporation model', is proposed. This model suggests that the interaction of the external electric field with the lipid dipoles of a pore configuration induces and stabilizes permeation sites, enhancing cross-membrane transport. The study shows that the electrically mediated DNA transfer into LTK⁻ cells strongly depends on the initial field strength of the electric impulses. A threshold of ~6-7 kV/cm and an optimum field strength range of 8 (±0.5) kV/cm are required for DNA transfer leading to colony formation in HAT selection medium. Higher field strengths irreversibly damage the cells. The presence of divalent ions like Mg²⁺ increases DNA binding to cell surfaces but reduces gene transfer. The optimal conditions for gene transfer include the absence of Mg²⁺, three pulses of 8 (±5) kV/cm and a pulse decay time of ~5 μs at 20°C, and transferring the pulsed sample into selection medium 10 minutes after pulsing. High cell density and DNA concentration are also important for efficient gene transfer. The study also explores the biophysical aspects of DNA transport through cell membranes. The mechanism of cross-membrane transport of DNA is either an unspecific membrane process or may be specifically mediated by permeases activated in high electric fields. A biological membrane is a co-operatively stabilized organization of lipids and proteins containing dynamic, locally limited structural defects. These defects are candidates for the onset of electric-field induced perturbations, leading to permeation sites for enhanced material exchange. The presence of an external electric field favors charge and dipole configurations that lead to larger dipole moment components in the field direction, potentially thinning membrane patches and forming holes. The study provides a thermodynamic and kinetic analysis of electric field-induced changes in membrane structure. The results show that the formation of colonies of LTK⁺ cells in the selection medium is due to DNA penetration through permeation sites induced by the electric field. The steep dependence of colony density on field strength reflects a steep dependence of θ on E,This study describes a method for gene transfer into mouse L cells using electroporation in high electric fields. Electric impulses (8 kV/cm, 5 μs) significantly enhance DNA uptake into cells. When linear or circular plasmid DNA containing the herpes simplex thymidine kinase (TK) gene is added to a suspension of mouse L cells deficient in the TK gene and then exposed to electric fields, stable transformants are formed that survive in HAT selection medium. At 20°C, after three successive electric impulses followed by 10 minutes for DNA entry, 95 (±3) transformants per 10⁶ cells and per 1.2 μg DNA are obtained. This method is simple, easily applicable, and highly efficient compared to biochemical techniques. The mechanism of DNA transport through cell membranes is not fully understood, but a physical model, the 'electroporation model', is proposed. This model suggests that the interaction of the external electric field with the lipid dipoles of a pore configuration induces and stabilizes permeation sites, enhancing cross-membrane transport. The study shows that the electrically mediated DNA transfer into LTK⁻ cells strongly depends on the initial field strength of the electric impulses. A threshold of ~6-7 kV/cm and an optimum field strength range of 8 (±0.5) kV/cm are required for DNA transfer leading to colony formation in HAT selection medium. Higher field strengths irreversibly damage the cells. The presence of divalent ions like Mg²⁺ increases DNA binding to cell surfaces but reduces gene transfer. The optimal conditions for gene transfer include the absence of Mg²⁺, three pulses of 8 (±5) kV/cm and a pulse decay time of ~5 μs at 20°C, and transferring the pulsed sample into selection medium 10 minutes after pulsing. High cell density and DNA concentration are also important for efficient gene transfer. The study also explores the biophysical aspects of DNA transport through cell membranes. The mechanism of cross-membrane transport of DNA is either an unspecific membrane process or may be specifically mediated by permeases activated in high electric fields. A biological membrane is a co-operatively stabilized organization of lipids and proteins containing dynamic, locally limited structural defects. These defects are candidates for the onset of electric-field induced perturbations, leading to permeation sites for enhanced material exchange. The presence of an external electric field favors charge and dipole configurations that lead to larger dipole moment components in the field direction, potentially thinning membrane patches and forming holes. The study provides a thermodynamic and kinetic analysis of electric field-induced changes in membrane structure. The results show that the formation of colonies of LTK⁺ cells in the selection medium is due to DNA penetration through permeation sites induced by the electric field. The steep dependence of colony density on field strength reflects a steep dependence of θ on E,