Insulin-like growth factor binding proteins (IGFBP) are modulators of insulin-like growth factor I (IGF-I), which functions as a regulator of cartilage and bone development. Rapid growth and high starch diets have been associated with increased circulating concentrations of IGF-I, which lead to developmental orthopedic disorders in foals. The objective of this study was to assess the effects of age, diet, growth and season on plasma IGFBP and IGF-I concentrations from birth to 16 mo of age in Thoroughbred foals. Twenty-two mares maintained on mixed grass/legume pasture were randomly divided into two dietary groups and fed either a high starch and sugar supplement (SS) or a starch-restricted fiber and fat supplement (FF) for 3 mo prior to and after foaling. Monthly blood samples were obtained from SS and FF foals up to 16 mo of age and analyzed for IGF-I using an RIA and IGFBP using western ligand blot analysis. Auxilogical measurements of foals were also obtained each month. The effect of diet, month, and diet*month interactions upon the subject horse (diet) were analyzed using a mixed model with repeated measures, and correlations of normally distributed data were calculated using Pearson’s correlation. Six IGFBP bands of molecular weights 109, 39, 36, 35, 34, and 33 kDa were identified in foal plasma. Doublet bands were recognized at 109, 39, and 35 kDa, however they were not all believed to be singular pure IGFBP. A band with a molecular weight of 213 kDa was observed and presumed to be a ternary complex of IGFBP-3, IGF-I, and an acid labile subunit. The IGFBP 109 kDa has been previously recognized as a band unique to the equine, it was not a singular pure IGFBP because of its high molecular weight. No effect of diet on plasma IGFBP was found in individual sampling of yearlings, but an effect of month was noted when testing May - August 2001 against May - August 2002 in pooled plasma samples with concentrations of the IGFBP 39 kDa increasing (P < 0.0003). In contrast, concentrations of the IGFBP’s 33, 34 and 36 kDa decreased (P < 0.003, P < 0.0002, and P < 0.0003 respectively). Environmental effects were noted upon IGFBP’s 33, 36, 39, and 109 kDa (P < 0.003, P < 0.001, P < 0.04, and P < 0.01) with a temperature*daylength interaction. Correlations existed between ADG and IGFBP 33 (r = 0.64; P < 0.0001), 34 (r = 0.40; P < 0.0001), 35 (r = 0.33; P < 0.0006), 36 (r = 0.47; P < 0.0001), and 39 kDa (r = - 0.18, P < 0.02). A correlation was also found between IGF-I and ADG (r = 0.11; P < 0.04), confirming the previously reported relationship of IGF-I in growth rate of foals. These results underline the importance of characterizing the activity of IGFBP’s in relation to growth, age and season when interpreting changes of the somatotropic axis. Further, the increase in certain IGFBP’s and simultaneous decrease in others stress the need for further research on the tissue specific modulating effects that IGFBP’s have on IGF-I.