Spontaneous and culture condition-dependent changes in P-glycoprotein expression and activity have been monitored in primary cultures of rat hepatocytes by using immunoblotting, PCR and fluorimetric techniques. In hepatocytes cultured in basal medium without addition of dexamethasone or 3-methylcholanthrene, mdr mRNA and P-glycoprotein increased progressively throughout a 72 h culture period, in concert with an enhancement in the ability to extrude the fluorescent dye Rhodamine-123. Addition of 1 microM dexamethasone to the culture medium slowed down the increase in mdr mRNA and P-glycoprotein, while inducing a significant increase in the efficiency of R-123 efflux. Addition of either 100 nM or 10 microM DEX produced different changes in mdr mRNA and protein, unrelated to the rate of Rhodamine-123 extrusion. When 50 microM 3-methylcholanthrene was added to the culture medium in the absence of any hormone supplementation, no significant changes in P-glycoprotein activity and expression took place, in comparison with control cultures. On the contrary, in the presence of dexamethasone (100 nM and 1 microM), 3-methylcholanthrene induced an increase in mdr mRNA and in the amount of immunoblottable protein during culture, without producing any concomitant increase in the efficiency to extrude Rhodamine-123. The last phenomenon resulted to be an artefact, since 3-methylcholanthrene was shown to inhibit Rhodamine-123 transport competitively. These results indicate that rat hepatocyte P-glycoprotein may be variously modulated in vitro, by supplementing culture medium with hormones and/or xenobiotics. Functional activity of the P-glycoprotein is not necessarily related with protein amount and/or mdr RNA.