The rat is one of the most commonly used laboratory animals in biomedical research and the recent isolation of genuine pluripotent rat embryonic stem (ES) cell lines has provided new opportunities for applying contemporary genetic engineering techniques to the rat and enhancing the use of this rodent in scientific research. Technical refinements that improve the stability of the rat ES cell cultures will undoubtedly further strengthen and broaden the use of these stem cells in biomedical research. Here, we describe a relatively simple and robust protocol that supports the propagation of germ line competent rat ES cells, and outline how tuning stem cell signaling using small molecule inhibitors can be used to both stabilize self-renewal of rat ES cell cultures and aid evaluation of their differentiation potential in vitro.