We demonstrate that on the rat liver, testosterone (T) induced differentiated functions and enhanced unusual estrogen-binding protein (UEBP) content through mechanisms dependent on cell activation by androgens, the presence of growth hormone (GH) and the hormonal status of the animal. To determine whether liver cells are a target for androgens, we measured T effects on UEBP in gonadectomized adult male and female rats in vivo and in vitro. In ovariectomized rats, T increased 8- to 9-fold UEBP levels that remained constant during 10 days. Also in vitro, using hepatocytes from ovariectomized rats, T alone increased UEBP levels 3-fold in a dose-response pattern. Combining a fixed low dose of GH with different concentrations of T increased UEBP 2-fold above T alone. Whereas GH alone had no effects in ovariectomized rats, hepatocytes were responsive to GH, in a dose dependent pattern that was abolished when T was used together with GH. On the other hand, T alone had no effect in hypophysectomized-ovariectomized animals. The latter group was rendered T responsive after the simultaneous injection of GH with T that increased UEBP content 6.6-fold in vivo. Castrated males revealed a marked responsiveness to T and GH in vivo and in vitro, when added separately or in combination. The results obtained suggest a complex regulatory system and we conclude that T acts directly on rat liver as: (1) an inducer of sex differentiation; and (2) a regulator of UEBP production in males. In addition, liver regeneration studies in castrated-hypophysectomized males revealed the UEBP phenotype in daughter cells in the absence of treatment.(ABSTRACT TRUNCATED AT 250 WORDS)