Recent evidence from studies in both amphibians and mammals suggest that the fibroblast growth factor (FGF) family of signaling molecules and their receptors may play regulatory roles during early embryogenesis. We have used both standard and whole-mount in situ hybridization techniques to analyze the temporal and spatial expression patterns of the murine fibroblast growth factor receptor-1 (FGFR-1) in order to help define the role of FGFs in the processes of gastrulation and segmentation. FGFR-1 transcripts were detected in the primitive ectoderm of the egg cylinder embryo but not in the primitive endoderm or ectoplacental cone. During gastrulation, FGFR-1 mRNA were expressed at high levels in the migrating embryonic mesoderm of the mid-streak-stage embryo. Late-streak-stage embryos displayed strong expression in both the embryonic ectoderm and mesoderm. Within the ectodermal lineage, FGFR-1 mRNA later became localized to the neural ectoderm during its formation and continued to be expressed at high levels throughout neural development. In the mesodermal lineage, FGFR-1 transcripts became concentrated in the posterior medial mesoderm of the embryo as it condensed to form paraxial mesoderm. The most striking expression patterns were observed before and during segmentation where FGFR-1 was strongly expressed in the presomitic mesoderm and the rostral half of the newly formed somites. The patterns of expression are consistent with a role for FGFR-1 in posterior mesoderm formation. FGFR-1 may also play significant roles in the formation of neural ectoderm and the early events that establish compartments within the developing somites.