Retinoid X receptor alpha (RXRalpha) can reveal diverse functions through forming a heterodimer with peroxisome proliferator-activated receptor alpha (PPARalpha). However, the mechanism of regulation of the cellular RXRalpha level is unclear. Thus, quantitative change of RXRalpha was investigated in mouse liver. Nuclear RXRalpha level was constitutively lower in PPARalpha-null mice than in wild-type mice. The level was also increased by clofibrate treatment in wild-type mice without a concomitant increase of RXRalpha mRNA, but not in PPARalpha-null mice. Pulse chase experiments demonstrated that the presence of PPARalpha and its activation by ligands significantly affected the stability of nuclear RXRalpha. These findings suggest a novel regulatory mechanism of nuclear RXRalpha in vivo.