The constitutively active receptor (CAR) is a crucial regulator of genes encoding for enzymes active in drug/steroid oxidation, conjugation, and transport. In our attempt to isolate the endogenous inhibitory ligand(s) for the mouse CAR, we found surprisingly that the inhibitory activity was associated with di- and tri-isopropylated phenyl phosphates that were present in livers of untreated mice. Trans-activation experiments in mammalian cells with synthetic compounds verified that mouse CAR was inhibited by various isopropylated phenyl phosphates (40-80%). Such triaryl phosphates are widely used as fire retardants, lubricants, and plasticizers, and some of them are known to disturb reproduction by currently unknown mechanisms. Equipped with the finding that these compounds could interact with mouse CAR, we proceeded to determine their functional effects on other nuclear receptors. Human CAR and pregnane X receptor (PXR) were variably activated (2-5-fold) by triaryl phosphates while mouse PXR, peroxisome proliferator-activated receptor-alpha, and vitamin D receptor were refractory. Among steroid hormone receptors, the human androgen receptor was inhibited by triphenyl phosphate and di-ortho-isopropylated phenyl phosphate (40-50%) and activated by di- and tri-para-substituted phenyl phosphates (2-fold). Our results add to the list of CAR and PXR activators and suggest steroid-dependent biological pathways that may contribute to the reproductive effects of triaryl phosphates.