Fgf-8 is a member of the fibroblast growth factor (FGF) family that was initially identified as an androgen-inducible growth factor in a mammary carcinoma cell line. Alternative splicing of the primary Fgf-8 transcript results in three messenger RNAs which code for secreted FGF-8 protein isoforms that differ only in their mature amino termini. Fgf-8 RNA is present from day 10 through 12 of murine gestation when analyzed by northern blot analysis, suggesting that Fgf-8 normally functions during post-gastrulation development. To characterize the temporal, spatial and isoform-specific aspects of Fgf-8 expression during mouse development, we performed in situ hybridization and ribonuclease protection assays between the days 8 and 16 of gestation. Fgf-8 expression is first detected at day 9 of gestation in the surface ectoderm of the first branchial arches, the frontonasal process, the forebrain and the midbrain-hindbrain junction. At days 10-12 of gestation, Fgf-8 expression is detected in the surface ectoderm of the forelimb and hindlimb buds, in the nasal pits and nasopharynx, in the infundibulum and in the telencephalon, diencephalon and metencephalon. Fgf-8 expression continues in the developing hindlimbs through day 13 of gestation but is undetectable thereafter. Ribonuclease protection assays reveal that RNAs coding for all three FGF-8 isoforms are present at days 10-12 of gestation. These results reveal a unique temporal and spatial pattern of Fgf-8 expression in the developing mouse and suggest a role for this FGF in multiple regions of ectodermal differentiation in the post-gastrulation mouse embryo.