The insulin-like growth factor binding proteins (IGFBPs) are polypeptides which are thought to modulate the bioactivity of IGF-I and IGF-II, and may also have activities independent of the IGFs. The expression patterns of IGFBPs-1, -3, -4, and -6 in midgestational rodents were analyzed using in situ hybridization to begin to characterize the role of these IGFBPs during development. All IGFBPs are expressed at least as early as rat embryonic day 14 (e14), and each has a unique pattern of expression. IGFBP-1 mRNA is expressed by the liver throughout mid and late gestation. IGFBP-3 mRNA is expressed at high levels in the urogenital tract, several muscle groups, and the nasal epithelia. IGFBP-3 transcripts are also expressed at lower levels by many non-neural tissue types, including the liver, stomach, and heart. IGFBP-4 is abundantly expressed by many tissues in the developing embryo, with the notable exceptions of the spinal cord, specific cartilage groups, and the thymic cortex. IGFBP-6 is expressed in the liver by e14, and also by a previously unrecognized cell population surrounding developing cartilage. Taken together these observations suggest distinct roles in development for each of the IGFBPs.