The circulating level of homocysteine (Hcy) is recognized as a major independent risk factor for cardiovascular disease in humans. Nonhuman primates are being investigated to see whether they will be accurate models for Hcy indicators of cardiovascular dysfunction. Normal reference values are available in humans for Hcy as classified by age, gender, ethnic origin, and biological factors, however similar information in nonhuman primates had not been published previously. The purpose of this report is to provide normal Hcy values in a large group of nonhuman primates in light of age, gender, and physiologic state (pregnancy and lactation) and to compare these values to the same parameters in humans to highlight similar and dissimilar trends. In addition, plasma levels of folic acid and vitamin B(12), which are determinants of Hcy status in humans, are presented. Samples obtained from a troop of 149 rhesus monkeys (Macaca mulatta) fed a high protein commercial diet were analyzed for Hcy by using high-performance liquid chromatography. Folate and vitamin B(12) levels were determined by using an autoanalyzer. Results (mean [95% confidence interval]) for the entire troop were: Hcy, 4.5 (4.2-4.9) micromol/L; folic acid, 8.6 (8.0-9.1) micromol/L; and vitamin B(12), 673 (611-741) pmol/L. Quantitative values are similar to published values for another species of wild-caught macaques. Similar to trends noted for humans, male monkeys had higher Hcy values than did female animals, pregnant animals had lower values than did nonpregnant ones, and Hcy levels were inversely proportional to plasma folate and vitamin B(12) concentrations. However, homocysteine levels in rhesus monkeys did not vary consistently with age, whereas they increase with age in humans.