In this study, we set out to assess whether the p53 protein affects mdm-2 gene expression as F9 embryonal carcinoma cells differentiate into parietal endoderm cells. It was previously reported that F9 cells possess abundant levels of wild-type p53 and upon induction to differentiate, p53 mRNA and protein levels decrease (Oren et al., 1982; Dony et al., 1985). We demonstrate that while p53 mRNA and protein levels decrease as F9 cells differentiate, mdm-2 mRNA and protein expression remains constitutive. Using RNA primer extension assays, we determined that the mdm-2 mRNA expression is not directed by p53 in either F9 embryonal (undifferentiated) or parietal endoderm (differentiated) cells. However, p53 protein does stimulate mdm-2 mRNA expression in response to u.v. irradiation. The inability of p53 to transactivate mdm-2 in undamaged F9 cells was not the result of latent pools as p53 sequence specific DNA binding activity was observed using electrophoretic mobility shift assays. Our results suggest that, in F9 cells, the p53:Mdm-2 autoregulatory loop is confined to pathways governing DNA damage.