In postnatal animals, most of insulin-like growth factor (IGF)-I and -II (IGFs) circulates in ternary complexes of 150 kDa composed of one molecule each of IGF-I or II, IGF binding proteins (IGFBP)-3, and an acid-labile subunit (ALS). Circulation of IGFs in 150 kDa complexes leads to their retention in the vascular system, prevents their hypoglycemic effects and is thought to promote their endocrine actions. Like IGF-I, hepatic synthesis of ALS is increased by growth hormone (GH). Thus ALS is an important physiological endpoint of GH signaling, but unlike the various IGFBPs and IGFs, ALS has received only limited attention. We have performed studies in both mice and sheep in order to understand the regulation of ALS synthesis and its role in the circulating IGF system. First, we have cloned the mouse and sheep ALS genes and shown that they are organized similarly, with 2 exons and a single intron. Second, we have identified transcription factors binding to two proximal promoter cis-elements that are important to the basal and GH regulation of the mouse gene. We have broadened the relevance of these studies by showing that these two elements are conserved in the sheep and human ALS promoters. One interesting species difference is that ALS gene expression is increased after birth more rapidly and abruptly in the sheep than in the mouse. Finally, an ALS knockout model was created by inactivating the ALS gene in mouse embryonic stem cells. Mice that are homozygous for the mutation grow at a slower rate after birth. This growth depression is associated with large decreases in the plasma concentrations of both IGF-I and IGFBP-3, indicating the critical role ALS plays in regulating the circulating levels of these proteins. Studies of this model will lead to a better understanding of the role of ternary complexes in growth, development and diseases.