The current study was undertaken 1) to develop insulin-like growth factor-I (IGF-I) and IGF-binding protein-3 (IGFBP-3) radioimmunoassays (RIAs) for porcine serum and 2) to investigate, by using these RIAs, the pattern of serum concentrations of IGF-I and IGFBP-3 and correlation between these two variables in growing pigs. Commercially available IGF-I antigen and its antiserum were used in IGF-I RIA. Porcine IGFBP-3 (pIGFBP-3) antigen was purified by SDS-PAGE and electro-elution from total serum IGFBPs that had been purified by cation-exchange chromatography and IGF-I affinity chromatography. pIGFBP-3 antiserum was raised in a New Zealand White rabbit. RIAs were performed by the standard double-antibody method using ^(125)I-labeled antigen. The validity of each RIA was demonstrated by confirming the parallelism of dose-response displacements of the ^(125)I-labeled antigen from its corresponding antibodies by unlabeled antigen and porcine serum. Blood samples were taken from gills and barrows, eight animals per sex × age, at 0 (birth), 1.5, 3, 4 and 5 months of age. Serum IGFBP-3 concentration increased within a 1.5-fold range during the postnatal development in gills, whereas in barrows, it was stable during the same period. Mean IGFBP-3 concentration was not different between the two sexes. Mean IGF-I concentration was ∼15% greater in gills than in barrows (237 vs 203ng/㎖; P$lt;0.05). In slight contrast to the IGFBP-3 developmental pattern, IGF-I concentration increased by 3∼5 fold during the postnatal development. Nevertheless, concentrations of IGFBP-3 and IGF-I were positively correlated in gills (r = 0.41; P = 0.01), but not in barrows (r = -0.14; P = 0.43). In summary, we have developed IGFBP-3 and IGF-I RIAs for porcine serum which can provide a useful experimental tool for future studies of circulating porcine IGF system. Moreover, results of the present study suggest that gonadal hormones also may regulate the secretion of IGFBP-3 and IGF-I in the pig.