The change in transfection efficiency of electroporation by the combined treatment with mild preheating (40A degrees C for 30 min) was investigated. HCT 116, HeLa S3 and SGC 7901 cells were treated with electroporation in medium containing pBKCMV-Luc plasmid with or without preheating. After 24 h, luciferase activity was increased by 36, 28 and 77%; luciferase mRNA transcription was increased by 45, 50 and 68%; and fluorescein isothiocyanate-dextran accumulation was increased by 9, 35 and 15% in preheated groups, respectively. These results demonstrate that the transfection efficiency was enhanced by mild preheating. The mechanism partially involves increased macromolecular particle accumulation.