We have looked at the relationship between fasting levels of insulin and a small insulin-like growth factor (IGF)-binding protein (IBP-1) in a cross-sectional study of 116 normal subjects aged 5-48 years. The relationship between IBP-1 and insulin was also examined within individual normal children in overnight profiles of IBP-1 and insulin obtained from two children at each stage of puberty (Tanner stages 1-5). In the cross-sectional study high levels of IBP-1 were found in early childhood and these fell throughout puberty as fasting levels of insulin rose. Multiple regression analysis revealed that both these changes were predominantly due to pubertal development rather than to age. After the age of 16 IBP-1 levels remained low despite fasting insulin levels returning to prepubertal levels. A strong negative correlation was obtained between IBP-1 and insulin in children of 5-16 years (r = -0.63; n = 60; P less than 0.001), no such relationship being found after the age of 16. In the second study, IBP-1 underwent a marked circadian variation in all cases and an inverse correlation with insulin, measured at the same time, was obtained at pubertal stages 1 to 4, but not at stage 5 (pooled data stages 1-4, r = -0.69; n = 53; P less than 0.001). We have demonstrated that a potential inhibitor of IGF-activity is inversely related to insulin throughout the period of active GH-related growth and that this relationship weakens after puberty.