A radioimmunoassay (RIA) for the rat growth hormone binding protein (GHBP) was developed using a synthetic peptide (corresponding to the hydrophilic carboxyl-terminal sequence of mouse GHBP) as standard and a monoclonal antibody (MAb 4.3) reactive with this peptide as the primary antibody. The values for GHBP concentration obtained for normal rats using this assay compare favourably with those obtained by gel filtration and ELISA methods. The concentration of GHBP in normal male rats at 11 weeks of age (680 +/- 30 ng/ml, SEM, n = 9) was significantly less than the concentration in normal females (943 +/- 47 ng/ml, SEM, n = 25). In 11-week-old dwarf male rats the concentration of GHBP was 423 +/- 35 ng/ml (n = 8); less than in dwarf females (542 +/- 32, P < 0.05, n = 9) and normal males (680 +/- 30, P < 0.001, n = 9). The GHBP concentration in dwarf rats was not age-dependent, whereas in normal females the concentration of GHBP increased with age. The availability of an RIA which is not susceptible to interference by endogenous GH, will facilitate further studies on hormonal and nutritional regulation of the rat GHBP. The assay was applied to studying the effects of IGF-I infusion (240 micrograms/day for 1 week) and GH injection (65 micrograms/100 g body weight, twice daily for 1 week and 4 weeks) on the serum concentration of GHBP in 11-week-old Lewis dwarf rats. Hepatic GH binding sites were also measured in desaturated membranes from the same animals.(ABSTRACT TRUNCATED AT 250 WORDS)