The plasma low density lipoproteins (LDL) of rhesus monkeys fed 3 atherogenic diets exhibited thermal transitions at temperatures much higher (37--43 degrees C) than those observed in control animals or in normal humans (20--33 degrees C). The same differences were noted in the neutral lipids (cholesteryl esters and triglycerides) which were isolated from the respective lipoproteins. In particular, the difference in thermal properties between the normal and abnormal LDLs was attributable to subtle differences in their cholesteryl ester compositions (mainly an increase in the saturated and monounsaturated fatty acid moieties), with altered triglyceride contents playing only a minor role. Thus, at body temperature, the hyperlipidemia that follows the administration of atherogenic diets is associated with a high degree of order of the neutral lipids in the core of the LDL particle. This, in turn, may be related to the atherogenicity of the abnormal lipoprotein species.