Objective Using the concept of allostatic load(AL)to evaluate aging of male SD rats and the effectiveness of Zuogui Pill in naturally aging rats.Methods Naturally aging male SD rats were tested at the ages of 2,5,8,14,18,and 21 months.They were divided into an elderly control group,low-dose Zuogui Pill group,and high-dose Zuogui Pill group.Intervention with Zuogui Pill was trialed for 3 months.Blood samples were taken from the tails of rats each month,and the number of T lymphocytes and rate of apoptosis were measured by flow cytometry.Low-density lipoprotein cholesterol(LDL-c),high-density lipoprotein cholesterol(HDL-c),triglycerides(TG),total cholesterol(TC),free fatty acids(FFA),25 hydroxyvitamin D(25-OH-D),corticosterone(CORT),C-reactive protein(CRP),and interleukin-6(IL-6)were detected in rat sera.By identifying the collinearity between indicators and professional considerations,LDL-c,TC,HDL-c,FFA,TG,CORT,IL-6,CRP,25-OH-D,CD3+T cell count,and CD3+T cell apoptosis rate were included in the AL scoring.The threshold for each indicator was established with data from 5-month-old rats,and the score was 1 point below or/and above the threshold.Results The serum levels of LDL-c,TG,TC,25-OH-D,CRP,and IL-6 of rats showed significant changes with age,although the patterns of change differed.The CD3+T lymphocyte count significantly decreased with age(P＜0.01),while the apoptosis rates of CD3+,CD4+,and CD8+T lymphocytes significantly increased with age(P＜0.01).Zuogui Pill significantly increased serum CORT levels in elderly rats(P＜0.01)and reduced the apoptosis rate of CD8+T lymphocytes(P＜0.05).The AL score began to increase in rats at 5 months of age and reached its peak in those of 18 months of age.Conclusions AL can better characterize the aging process compared to a single indicator.Zuogui Pill can improve the stress response ability of aging rats and alleviate immunosenescence.