The present study was designed to determine whether normolipidemic male squirrel monkeys ( Saimiri sciureus) exhibit low density lipoprotein (LDL) heterogeneity similar to that observed in humans and if present, whether LDL subfractions are altered by consumption of low vs. high dose ethanol (EtOH). Primates were divided into three groups designated control, low, and high EtOH and fed isocaloric liquid diets containing 0%, 12% and 24% of calories as EtOH, respectively, for 6 months. The 12% EtOH caloric level resulted in a modest, non-significant increase in high density lipoprotein (HDL) cholesterol and no change in LDL cholesterol or plasma apolipoprotein B (apo B), while the 24% dose produced significant elevations in plasma, LDL and HDL cholesterol and apo B. Using a single-spin density gradient ultracentrifugation procedure developed for humans, three distinct LDL subclasses designated LDL 1a ( d = 1.031 g/ml), LDL 1b ( d = 1.038 g/ml) and LDL 2 ( d = 1.046 g/ml) were isolated from all three treatment groups. Monkey LDL subfractions were nearly identical to very light, light and heavy LDL subspecies isolated from human plasma in terms of their: (1) isopycnic densities following ultracentrifugation; (2) co-migration as single bands with beta-electrophoretic mobility in cellulose acetate and agarose electrophoretic gels; (3) size-dependent migration pattern in polyacrylamide gradient electrophoretic gels; (4) co-migration as a single band corresponding to apo B-100, following SDS polyacrylamide gel electrophoresis; and (5) decrease in total cholesterol/protein ratios with increasing LDL subclass density. Although there were no treatment differences in LDL particle size, within each treatment group, mean particle size for each LDL subfraction was significantly different from every other subfraction. Low (12%) dose alcohol had no effect on LDL subfraction mass relative to controls while high alcohol consumption resulted in marked increases in all lipid (except triglyceride) and protein of the larger, buoyant LDL subspecies (LDL 1a and LDL 1b). Moreover, the best correlation between plasma apo B and LDL subfraction total mass was demonstrated with LDL 1b ( r = 0.735). Since neither the lipid nor the protein concentration of the small, dense, purportedly more atherogenic, LDL 2 changed with the 24% EtOH dose, we propose that the LDL subfraction alterations associated with high alcohol intake in squirrel monkeys (increased LDL 1a, increased LDL 1b, LDL 2 no effect) may represent a compensatory response to modulate the overall atherogenic lipoprotein profile associated with elevations in total LDL cholesterol and plasma apolipoprotein B.