This study examines the production and characteristics of insulin-like growth factor I (IGF I) and its binding protein (BP) from adult rat hepatocytes in primary culture. IGF I and BP in samples of conditioned medium were separated by automated high pressure gel permeation chromatography at pH 2.8, with total recovery of both species. Measured by a specific RIA, with about 2% IGF II cross-reactivity, the rate of production of IGF I [compared with a human IGF I (hIGF I) standard] was 4.39 +/- 0.57 pmol/mg cell protein X 24 h; measured by a radioligand assay using radioiodinated hIGF I as tracer, BP was produced at a rate of 2.97 +/- 0.54 pmol binding sites/mg cell protein X 24 h. High pressure permeation chromatography studies indicated approximate mol wts of 7,500 for hepatocyte IGF I and 50,000 for BP. The pI of hepatocyte IGF I was 8.8, and of the predominant BP species, 6.8. Rat serum treated in the same way as hepatocyte medium showed IGF I and BP activities at identical pI values to those from hepatocytes but, in addition, contained BP activity at more acidic pI values, apparently not produced directly by the liver. Competitive binding studies using acid-chromatographed hepatocyte or serum BP, and hIGF I and IGF II as labeled and unlabeled ligands, also suggested that BP activity in serum was heterogeneous, while BP from hepatocytes appeared to be a single species with similar affinities for IGF I and IGF II. It is concluded that adult rat hepatocytes produce immunoreactive IGF I, pI 8.8, and BP, pI 6.8, which are released into the circulation, but that additional, more acidic, BP species which are also found in serum could result from extrahepatic production or processing.