Technical Abstract: The current study examined the effects of feeding soy protein isolate (SPI) and rice protein isolate (RPI) on insulin sensitivity and fat breakdown in weanling rats consuming high fat/high cholesterol diets. Male Sprague-Dawley rats were placed on semi-purified diets containing the milk protein casein (CAS) as the sole protein source on PND24 or onto CAS, SPI-, or RPI-based Western diets made with high fat and supplemented with 0.5% cholesterol. Prior to sacrifice at age 50 days, an oral glucose tolerance test was performed. High fat/high cholesterol feeding impaired glucose disposal (p < 0.05), suggesting insulin resistance. This was prevented by feeding SPI- or RPI-containing diets (p < 0.05). Increased serum and liver fat and cholesterol levels occurred with high fat/cholesterol feeding. Both SPI and RPI reduced serum and liver cholesterol (p < 0.05) and liver fat levels (p < 0.05). Expression of liver genes regulated by several transcription factors (PPAR'-, PPAR'- and LXR') were increased (p < 0.05) by feeding SPI- and RPI-containing diets relative to those in CAS-fed rats, as determined by real time RT--PCR. This was accompanied by increased binding of the transcription factors to their response elements on the acyl Co-A oxidase, glucokinase, and CYP7A1 gene promoters as measured in EMSA and ChIP assays. Increased expression mRNA and protein encoding then transcription factors themselves was also observed (p < 0.05). These data suggest that SPI and RPI increase insulin sensitivity via increased PPAR' signaling, and improved fat and cholesterol regulation as a result of signaling via PPAR' and LXR.