INTRODUCTIONThis protocol describes a method for the production of transgenic rats by coinjecting circular PiggyBac transposon-containing plasmid DNA with transposase-encoding mRNA. After fertilized eggs are collected from females, pronuclei are microinjected with the DNA and mRNA and then transferred immediately into the oviducts of pseudopregnant foster females. Using this procedure, we have obtained transformation frequencies of 33%-100% (average of ~80%). Traditional methods for generating transgenic rats (i.e., microinjection of linear DNA) yield 3%-41% transgenic founders. Thus, transposon-mediated transformation of fertilized rat eggs is an efficient alternative method to generate transgenic rats.