Rats given a choice between fluid containing 0.1-0.5% triglyceride oil and the same fluid without oil, generally preferred the fluid containing oil. Several experiments indicate that this preference is based on the detection of impurities rather than triglycerides per se. Rats preferred crude triolein to a greater degree than they did highly purified triolein or corn oil. Rats did not show any preference for or aversion to tristearin, a fat that does not decompose as readily as triolein. Rats that have been trained to avoid a dilute suspension of triolein, also avoided an aqueous extract of triolein. Since rats that had been trained to avoid triolein oil also avoided corn oil, it seems likely that different oils may possess similar impurities. Since training rats to avoid mineral oil did not reduce preference for triolein, these substances may have different flavors. It is proposed that rats use fat decomposition products to detect the presence of fats in foods.