The estrogen receptors (ERs) are ligand-inducible transcription factors that play key roles in the control of growth and differentiation in reproductive tissues. We showed that the novel Dbl family proto-oncoprotein Brx enhances ligand-dependent activity of ER via a Cdc42-dependent pathway. Brx also significantly enhances ligand-dependent activity of ER. This enhancement is not affected by inhibition of p44/42 mitogen activated protein kinase (MAPK) activation by PD98059. However, addition of the p38MAPK inhibitor SB202190 abrogates the enhancement of ER activity by Brx, showing that p38MAPK activity is required for the enhancement of ER function by Brx. In COS-7 cells, transfection of Brx leads to activation of endogenous p38MAPK activity. Co-expression of the 2 isoform of human p38MAPK and a constitutively active form of the p38MAPK kinase MKK6 (MKK6-EE) synergistically augments ligand-dependent activity of ER. Our findings suggest that p38MAPKs may be important regulators of ER activity.