In many situations, it is important to quantitate accurately expression of a transiently transfected gene. This is most often the case when mechanisms controlling gene expression are under study, and the roles of several possible promoter and enhancer elements and their interacting factors are to be assessed. Accurate quantitation of expression driven by these different elements requires a gene-transfer procedure that will efficiently and reproducibly deliver the reporter construct into a relatively high percentage of the cells being transfected (1 ,2 ). In addition, the reporter assay should give high sensitivity, reproducibility, and linearity of response to increasing concentration of construct, and have a wide dynamic range. The combination of transfection by electroporation with a calorimetric or photometric reporter gene assay meets these requirements, and provides a rapid and sensitive method for the study of genetic regulatory elements in many types of cells.