Adult male rhesus monkeys representing a wide range of adiposity were characterized according to body dimensions, eating behaviors, and endocrinologic/metabolic indices. Body fat, located most prominently on the abdomen, ranged from 30 to 61% of body weight. Three groups were formed on the basis of body composition data: Very Obese (VO), Moderately Obese (MO) and Nonobese (N). These groups did not differ in food intake, preferences for sweet solutions, or compensatory changes in food intake following consumption of sugar solutions. Glucose tolerance was normal, but fasting insulin levels and insulin response to glucose loading increased with increasing adiposity. Fasting triglyceride levels were highly correlated with insulin values and body fat. It is concluded that some adult male rhesus monkeys develop obesity without obvious differences in eating behavior and that obese monkeys, like obese humans, are at risk for diabetes mellitus and its complications.