The insulin-like growth factor-binding proteins (IGFBPs) comprise at least six distinct species that may modulate the action of IGFs. IGFs are important regulators of fetal growth and differentiation. To define sites of IGFBP mRNA synthesis, we have used in situ hybridization techniques in mouse conceptuses of different gestational ages (11-18 days). Expression of mouse (m) IGFBP-1 was detected in mouse conceptuses after day 12 of gestation and was restricted to the liver. Transcripts for mIGFBP-2, -4, and -5 were detected in various tissues and were found in all stages tested. In contrast, expression of mIGFBP-3 and -6 could be only weakly detected in late gestational conceptuses. Comparison of the expression patterns of mIGFBP-2, -4, and -5, which were found widely distributed in mouse conceptuses, revealed that mIGFBP-2 was expressed in the mesoderm-derived part of the tongue (day 13.5), but mainly in the ectodermal layer. Transcripts for mIGFBP-4, however, were detected only in the mesodermal part, whereas expression of mIGFBP-5 was restricted to the ectodermal layer. A similar distribution pattern was observed in the lung (day 18). In general, expression of mIGFBP-2 and -5 was detected in the same cells, whereas mIGFBP-4 and -5 were expressed mainly in different cell types. These data suggest that the different mIGFBPs might play distinct roles in mouse embryonal and fetal life.