One hundred four adult male rats, which had been fed a commercial rat diet from weaning, were randomly allotted to four dietary groups, 26 rats per group. Two randomly selected rats from each group were sacrificed at the following days on experiment: 0, 7, 9, 11, 13, 15, 18, 22, 30, 36, 42, and 48. The eight remaining rats (which were fed the diets to provide replacements in case of animal loss) were not used. The objective of the experiment was to determine the effects of sources of dietary fat and protein on tissue cholesterol levels and on cholestero-genesis in the liver and small intestine. Experimental diets were isocaloric and provided 38 and 19% of the calories from fat and protein, respectively. The principal fat and protein sources for the four respective diets were (1) soybean oil and soy protein isolate, (2) soybean oil and casein, (3) beef tallow and soy protein isolate, and (4) beef tallow and casein. The commercial rat diet contained no cholesterol whereas all four experimental diets were standardized to 0.2% cholesterol;Blood plasma cholesterol levels of rats fed soybean oil (polyunsaturated fat) were 10% lower than those of rats fed tallow (saturated fat). Furthermore, plasma cholesterol levels of rats fed soy protein (plant protein) were 13% lower than those of rats fed casein (animal protein). The eight rats sacrificed at the beginning of the experiment (0 days) had, on the average, lower plasma cholesterol levels than rats fed any of the experimental diets;Liver cholesterol levels were higher for rats fed soybean oil or casein protein than for those fed tallow or soy protein isolate. Cholesterol content of the intestinal wall, however, was not significantly affected by diet. Furthermore, dietary fat and protein source had no significant effect on hepatic cholesterogenesis; however, intestinal cholesterogenesis was higher in animals fed beef tallow and casein. The tallow plus casein diet demonstrated a synergistic response in the small intestinal cholesterol synthetic rates. Intestinal cholesterogenesis of the eight rats sacrificed at 0 days was greater, on the average, than of rats fed any of the experimental diets;These results suggest that the hypocholesterolemic action of unsaturated fat and soy protein may be related to a lowered intestinal cholesterogenesis. In addition, dietary polyunsaturated fat (soybean oil) caused a redistribution of cholesterol from plasma to liver.