To elucidate the role of IGF-1 in the galactopoietic effect of somatotropin, the partition of plasma IGF-1 between groups of binding proteins (BP) was studied in goats at different stages of pregnancy, lactation and somatotropin treatment. By size-exclusion chromatography, total plasma IGF-1 was divided into a 150 kDa class and a 35 kDa class. Lactation was associated with marked reduction in plasma concentrations of IGF-1, specifically of the 150 kDa class. During lactation, BP3 concentrations were also low and BP3 circulated mainly in a "free"; or non-complex bound form. Somatotropin treatment changed IGF-1 concentration and distribution towards that seen in pregnancy. It is concluded that the change in distribution of IGF-1 and its binding protein at the start of lactaion means increased turnover of IGF-1 and probably increased availability of IGF-1 to mammary cells and other target tissues. The observed changes may be important aspects of the homeorhetic adaptation to lactation.