Insulin-like growth factor (IGF) circulates in blood in two large molecular mass forms of 150 and 40 kD. Under normal conditions, most of the IGF is bound to the 150-kD complex by which it is retained in the circulation and therefore unable to exert acute insulin-like actions. The aim of this study was to answer the question whether or not IGF in the 40-kD complex is bioavailable to insulin target tissues and thus can cause acute insulin-like effects in vivo. Intravenously injected 1:1 molar recombinant human (rh) IGF I/rhIGF binding protein (BP)-3 complex lowered blood glucose and stimulated glycogen synthesis in diaphragm of hypophysectomized, but not of normal rats. The serum half-lives of the two components of the complex were similar to each other, but considerably shorter in hypox than in normal rats. On neutral gel filtration of serum both components of the injected complex appeared predominantly in the 150-kD region in normal rats. In hypox rats which lack the 150-kD complex they were found in the 40-kD region and disappeared rapidly from the circulation. We conclude that in the absence of the 150-kD complex, IGF associated with the 40-kD complex can rapidly leave the vascular compartment, reach insulin or type 1 IGF receptors and exert acute insulin-like effects.