RIA for insulin-like growth factor-I (IGF-I) was performed on Tris-neutralized, acid-ethanol extracts of porcine, bovine, ovine and human mammary secretions, and porcine maternal and neonatal sera. High levels (50-500 ng/ml) of immunoreactive IGF-I were present in the colostrum of all three animal species. IGF-I was also identified in porcine milk, though at levels 10- to 100-fold reduced relative to that in colostrum. Maternal (pig) sera was characterized by IGF-I concentrations intermediate between that in colostrum and that in milk. IGF-I levels were relatively low in serum of newborn pigs and exhibited an approximately 1.4-fold increase between Days 3 and 7 of postnatal life. Fractionation of pig colostrum in nondenaturing, gel-filtration columns demonstrated association of endogenous IGF-I with two prominent binding proteins (Mr's of 150,000 and 50,000 for the complexes). A third immunoreactive component was also observed to elute in the column void volume fractions (Mr greater than 158,000). The 150,000 and 50,000 Mr complexes were also present in serum obtained from sows at term. In contrast, IGF-I immunoreactivity in porcine milk was localized exclusively in the 150,000 Mr complex. Incubation of porcine colostrum and milk with 125I-IGF-I revealed a prominent, unoccupied IGF binding protein corresponding to that of the 150,000 Mr complex, whereas serum obtained from sows at term displayed both the 150,000 and 50,000 Mr unoccupied forms. Fractionation of (pooled) serum obtained from porcine neonates immediately at birth revealed a heterogeneous pattern of IGF-I immunoreactivity which included both the 150,000 and 50,000 Mr forms. Qualitative differences in this chromatographic pattern were apparent in serum at 6 hr postnatal and after ingestion of colostrum had occurred. The unoccupied IGF binding proteins in newborn pig serum were solely of the small size class. These results demonstrate that mammary secretion of IGF-I and its binding proteins are temporally regulated during the period immediately surrounding parturition. Physiologic alterations in the serum IGF-I profile during early postnatal life may reflect in part the uptake and/or response of the neonate to maternal IGF-I.