Transgenic mice that constitutively overexpressed rat insulin-like growth factor binding protein-1 (IGFBP-1) were generated to determine the effects of overexpression of IGFBP-1 on growth and development. In offspring of three of the founders that showed high levels of expression, birth weight was significantly reduced to approximately 83-92% of the weight of their nontransgenic littermates. The transgenic mice gained less weight and were approximately 3.5-8 g lighter than nontransgenic littermates at 40 days of age. The difference in body weight between transgenic and wild-type mice was most apparent around the time of weaning when transgenic mice showed a more marked growth deceleration than wild-type mice. No significant catch-up growth was apparent over the first 3 months of life. In addition, offspring of all three high-expressing founders demonstrated fasting hyperglycemia. The transgene was highly expressed in the brain, uterus, lung, kidney, and heart, but little expression was detected in the liver. The weight of the brain relative to body weight was significantly reduced in transgenic mice compared with wild-type mice, whereas the relative weight of most other organs was similar to wild-type mice. These data demonstrate that IGFBP-1 may function to inhibit IGF action in vivo and that this inhibition selectively impairs development of organs such as the brain.