The relationship between serum levels of growth hormone (rGH), prolactin (rPRL), luteinizing hormone (rLH), follicle-stimulating hormone (rFSH), corticosterone, oestrogen, (oestradiol-17 beta) and testosterone and the hepatic binding sites specific to [125I]human-prolactin (h-PRL) were investigated in normal rats, in rats bearing the GH- and PRL-secreting tumour (GH3), and in rats 14 days after tumour removal. The presence of GH3 tumour elevated serum levels of rGH and rPRL and concomitantly increased the hepatic binding of [125I]h-PRL; the male rats had a greater increase than the female rats. The increased binding was due to an increase in the specific membrane binding sites, whereas the affinity constant (Ka) was not changed. In both male and female rats, there was a significant positive correlation between ser-m rGH levels (P less than 0.001) or serum rPRL (P less than 0.02) and specific binding of [125I]h-PRL in female rats only (P less than 0.02). In male rats, there was a significant negative correlation between serum testosterone levels and specific bindings of [125I]h-PRL (P less than 0.05). These results suggest that rGH and rPRL regulates the hepatic h-PRL receptors in female rats, and testosterone predominantly inhibits the induction of the hepatic lactogenic receptors in male rats.