A number of age-related changes in physiological functions have been identified in macaques and humans. However, few studies have examined physiological aging in chimpanzees, despite the increasing age of the chimpanzee population. We documented age-related changes in seven hematology and 17 clinical chemistry parameters in 49 adult chimpanzees (17 males, 32 females) as a comparative viewpoint with human and macaque aging. Longitudinal data were analyzed using weighted linear and quadratic mixed effects regression models. Male chimpanzees exhibited a significant age-related increase in anemia risk, based on significant decreases in hemoglobin (F1,49=12.45, P=0.0009) and hematocrit (F1,49=15.42, P=0.0003). Both sexes exhibited significant age-related decreases in both kidney and liver function. Decreases in kidney function were noted by significant increases in blood urea nitrogen (F1,45=3.92, P=0.036) and creatinine (F1,50=5.63, P=0.022) as well as changes in electrolyte (i.e., sodium, potassium, phosphorous, chloride) balance. Declining liver function was based on significant increases in globulin (F1,46=32.34, P<0.0001) and decreases in albumin (F1,48=23.42, P<0.0001). These changes were most evident beginning at 25–30 years of age in males and 30–35 years of age in females. We recommend amending chimpanzee age classes to categorize males over 25 years and females over 30 years as aged. Am. J. Primatol. 70:327–338, 2008. © 2007 Wiley-Liss, Inc.