Atherosclerosis and coronary heart disease are promoted by elevated serum low density lipoprotein cholesterol (LDL-C) and are retarded by increased high density lipoprotein cholesterol (HDL-C). Considerable variability in these lipoproteins has been observed in studies of captive animals subjected to extensive experimental manipulations, or by epidemiological studies of human beings. We have examined these variables in wild male baboons living undisturbed in their natural habitat in the Serengeti Ecosystem of East Africa. Among socially subordinate males, HDL-C and apolipoprotein A-I concentrations were significantly reduced by 31% and 25%, respectively, compared to concentrations in dominant individuals. There were no social rank differences in VLDL + LDL-C or its apolipoprotein (Apo B). Differences in age, sex hormone concentrations, rank-related diet, body weight, or gene pools were unlikely to explain this rank-related pattern. However, diminished HDL-C concentrations were associated with elevated basal cortisol concentrations, suggesting that exposure of subordinate individuals to elevated levels of social stressors could cause lower HDL-C concentrations.