It is well established that exercise affects immune responses in men, but little information is available for women. Whether exercise-induced immune responses are affected by circadian rhythm has not been determined. This study examined lymphocyte responses of women to exercise in the morning (AM) and evening (PM). Lymphocytes expressing markers for T, B, and natural killer (NK) cells were identified by FACScan in blood samples collected before (baseline), immediately after, and 40 min after exercise (recovery). Absolute counts of all lymphocyte subpopulations increased immediately after exercise, but at recovery values were below baseline counts. Circadian variations were noted in absolute counts for total T, helper/inducer T cells, and B cells. Although these counts were significantly higher in the PM, the magnitude of the immune response to exercise was the same in the AM and PM. In contrast, natural killer cells, both the relative counts and the magnitude of immune response were higher in the AM. Thus, it appears that despite differences in baseline cell counts the overall exercise-induced response in most lymphocyte subpopulations is similar in the AM and PM.