Fgf8 is a member of the fibroblast growth factor (FGF) family that plays an important role in early neural development. Cellular aggregation and retinoic acid (RA) are needed for mouse embryonic carcinoma (EC) P19 cell neural differentiation. We have examined the Fgf8 gene in P19 cells during neural differentiation and identified 2 alternatively spliced Fgf8 isoforms, Fgf8a and Fgf8b, among the 8 known splicing isoforms in mammals. The expression of Fgf8a and Fgf8b mRNAs transiently and rapidly increased in the early stage of P19 cells during RA-induced neural differentiation, followed by a decline in expression. The relative amount of Fgf8b was clearly higher than that of Fgf8a at different time-points measured within 24 h after RA treatment. Increased Fgf8b mRNA expression was cellular-aggregation dependent. The results demonstrated that cellular-aggregation-induced Fgf8b, but not Fgf8a, may play a pivotal role in early neural differentiation of P19 cells.