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The effect of feed (energy/protein) restriction on circulating concentrations of insulin-like growth factor (IGF)-I and II, and IGF-binding proteins (IGFBPs) was examined in young (4-week-old) chickens. Increasing levels of feed restriction caused progressive growth retardation, as evidenced by decreased body-weight gain and reduced bone growth. Plasma concentrations of both IGF-I and IGF-II were decreased, and the degree of reduction in the plasma concentrations of these growth factors appeared to be related to the magnitude of feed restriction. A tendency for greater decreases in these growth factors appeared to be associated with greater feed restriction at the majority of time points evaluated. However, nutritional restriction had a greater effect on plasma concentrations of IGF-I than on those of IGF-II. The reductions in plasma concentrations of IGF-I were observed earlier in the experiment and at a lower degree of nutritional deprivation than for plasma concentrations of IGF-II, possibly suggesting greater sensitivity of IGF-I plasma concentrations to feed restriction. Three IGFBPs with molecular weights of 30, 36, and 40 kDa were detected by radioligand assay following separation by SDS-electrophoresis. The 30-kDa IGFBP was most affected by feed restriction with binding activity of this IGFBP increased by 2 days of feed restriction irrespective of the degree of feed deprivation. The binding activity of the 36-kDa IGFBP was increased, albeit transiently, on the second day of feed restriction. Nutritional restriction had no discernible effect on the binding activity of the 40-kDa IGFBP. Increases in the binding activity of the 30-kDa IGFBP appeared to correspond with the observed decreases in IGF-I plasma concentrations. This suggests decreased bioavailability of IGF-I, and possibly IGF-II, attributed to the formation of a complex between IGF-I and the 30-kDa IGFBP during feed restriction. The initial increase in binding activity of the 36-kDa IGFBP may suggest that this binding protein also plays a role in the regulation and availability of circulating concentrations of IGF-I and IGF-II. Although the binding activity of the 40-kDa IGFBP was unaffected by feed restriction, we can not exclude its importance in the regulation of IGF-I. The substantial binding activity of the 40-kDa IGFBP observed in this experiment suggests that it is one of the major chicken IGFBPs, and that its role in IGF-I regulation warrants further study.