The insulin-like growth factors (IGF) circulate bound to specific high affinity binding proteins (IGFBPs) that modulate physiological responses to both IGF-I and IGF-II. Administration of bovine somatotropin (bST) to lactating cows has been shown to increase total serum levels of IGF-I; however, its effects on IGFBPs are unknown. Therefore, we determined the effects of bST on specific IGFBPs that are present in bovine serum and lymph. The results show that bovine serum contains IGFBPs with mol wt (Mr) estimates of 43,000, 39,000, 34,000, 29,000, and 24,000, as determined by ligand blotting. Using specific antisera, immunoblotting showed that the 43,000 and 39,000 Mr bands were IGFBP-3 and the 34,000 Mr band was IGFBP-2. All five forms of IGFBP were also present in afferent mammary lymph. To determine the effect of bST, four cows were treated with bST (40 mg/day) or vehicle for 12-day periods using a cross-over design. The intensity of the IGFBP-3 band increased 3.3 +/- 0.1-fold (mean +/- SE; P less than 0.01) by day 5 of bST treatment compared to that in controls. The intensity of the IGFBP-2 band decreased 3-fold. Serum levels of IGFBP-2, determined by RIA, decreased from 581 +/- 62 ng/ml during the control period to 156 +/- 52 ng/ml by day 5 of bST treatment (P less than 0.01). IGFBP-2 levels remained low for the entire 12-day treatment interval and rose to control levels within 4 days after cessation of bST. Results of a second study demonstrated that the decrease in IGFBP-2 concentrations in plasma observed during bST treatment (578 +/- 60 vs. 335 +/- 57 ng/ml) was paralleled by a decrease in IGFBP-2 levels in afferent mammary lymph (274 +/- 24 vs. 147 +/- 22 ng/ml). In contrast, the increase in IGFBP-3 levels observed in plasma during bST treatment by ligand blot analysis was not observed in lymph. In summary, bST increased serum levels of IGFBP-3 and decreased serum concentrations of IGFBP-2, while only IGFBP-2 levels were decreased in mammary lymph. Further studies are needed to determine whether these differences reflect differences in transport across capillaries or local production of specific forms of IGFBP.