The ontogeny of the somatotropin/insulin-like growth factor system was examined in well-fed pigs under basal conditions and during a short-term challenge of porcine ST (pST). The study was conducted with two replicates of eight castrate male pigs from 3.8 kg BW (10 d of age) to 92 kg BW (129 d of age). Pigs were reared individually with ad libitum access to milk replacer through 23 d of age. Thereafter, pigs were fed a corn, soybean meal, and dry skim milk diet formulated to exceed nutrient requirements by approximately 30%. Pigs were randomly assigned to receive daily i.m. injections of either 0 (buffer) or 120 microg/kg BW of pST for a duration of 4 d starting at 10, 19, 33, 43, 63, 83, and 125 d of age. Blood was collected via jugular venipuncture on d 0 and 4 of the challenge. Circulating levels of IGF-I were not dramatically affected by age, but levels of IGF-II were low from 10 to 19 d of age and then increased through later stages of growth. Circulating concentrations of IGF binding protein (BP)-3 increased with age (P < .05), but levels of IGFBP-2, a 30-kDa IGFBP, and IGFBP-4 were unchanged (P > .10). The pST challenge reduced plasma urea nitrogen at all ages, but the magnitude of the response was less in younger pigs compared with the maximum response in pigs greater than 30 kg BW (63 d of age). The IGF-I response to the pST challenge also increased from approximately 30% in young pigs to a threefold increase in older pigs. Regardless of age, concentrations of IGF-II were minimally affected by the pST challenge. Circulating levels of IGFBP-3 increased and IGFBP-2 levels decreased in response to the pST challenge, and the magnitude increased with age. The high nutritional status of pigs in the early phases of growth diminished the postnatal changes in IGF-I and IGFBP-2, but not IGF-II or IGFBP-3. Overall, data demonstrate a developmental regulation of the ST/IGF system, with pST challenges altering circulating concentrations of IGF-I, IGFBP-3, and IGFBP-2 coincident with changes in amino acid metabolism.