One hundred twenty-two wild red howler monkeys (Alouatta seniculus) were translocated during flooding of the forest at a hydroelectric dam site in French Guiana. Blood samples from 103 animals were evaluated for 13 hematologic and/or 22 serum chemistry parameters. Significant age-specific variation was found for white blood cell (WBC), lymphocyte, and platelet counts and for alkaline phosphatase values. Adult males and females had significant differences in red blood cell count, packed cell volume, and hemoglobin, creatinine, cholesterol, and calcium values. In juveniles, amylase and cholesterol levels were significantly lower in males than in females. Significant differences associated with reproductive status were also observed; i.e., lower cholesterol level in pregnant females and higher packed cell volume in lactating females. Chronic stress due to habitat disappearance may have been responsible for significant differences between thin animals and those in good condition. Thin animals had lower WBC, eosinophil, basophil, and monocyte counts and higher platelet counts. Capture stress was probably responsible for high and variable levels of creatine kinase, aspartate aminotransferase, and lactate dehydrogenase.