Rhesus monkeys (seven females and six males) were fed ad libitum a diet comparable to that consumed by humans, containing 25% by weight fat (40% of calories) and 0.15% cholesterol (0.3 mg/kcal) for 12 months (phase 1). From the 13th month all monkeys were continued on the diet while four females and three males were given 50% coffee as their fluid intake and the remainder were water controls (phase 2). Major changes in total plasma lipids and lipoprotein profiles occurred within 3 months and generally plateaued thereafter. Total plasma protein remained constant while total plasma cholesterol, phospholipids, triglycerides, and free fatty acid levels increased. After the initiation of coffee, there was no difference between the diet-water group and the diet-coffee group with respect to total protein and lipids in plasma. Total concentrations of plasma lipoproteins varied throughout the study. The pre-beta (very low density lipoproteins) fraction remained constant while the beta (low density lipoprotein) and alpha (high density lipoprotein) fractions, along with their major components, increased within 3 to 6 months, then fluctuated in absolute weight thereafter. The introduction of 50% coffee was without effect on the dynamic changes in these fractions as fluctuations were found in both groups and to the same magnitude. This leads the authors to suspect that such variations are "normal" or are in response to the diet per se. The atherogenic diet induced fatty streaks in the aortas of all monkeys to a greater extent in males than females. However, there were no gross differences in the quantity or distribution of the streaks between control and coffee-drinking monkeys.