Two fibroblast growth factor (FGF) receptor substrates (FRS2 and FRS3) are involved in downstream signaling from activated FGF receptors and neurotrophin-activated Trk receptors. Despite the importance of signaling from these factors in embryogenesis, FRS2 and FRS3 expression patterns during development are unknown. In this study we characterize the expression of FRS2 and FRS3 from E7 to parturition and in adult murine tissues. Both are first detected in whole E8.5 CD1 mouse embryos. FRS2 is detected as early as E7 in the developing syncytiotrophoblast, later in the neural tube (NT) and in many adult and fetal tissues. FRS3 is more restricted in location than FRS2 (fetal NT, heart, stomach, liver and some adult tissues), and is expressed predominantly in the ventricular layer of the developing NT and brains of murine embryos.