Immune functions generally decline with aging. However, the onset and the rate of the functional decline may be different in each lymphoid compartment. We studied the effect of aging on the murine Peyer's patch (PP) cells, and mesenteric lymph node (MLN) cells, which are a part of gut-associated lymphoid tissues (GALT). The capacity of proliferative responses to mitogens of lymphoid cells from GALT decreased with aging. However, the rate of the decrease was much slower than that in the spleen cells. Production of interleukin-2 (IL-2) and interleukin-3 (IL-3) in aged T cells was also studied. IL-2 production of T cells from PP, MLN, spleen cells decreased with age. The age-related decrease was observed at 21 months of age in spleen and at 24 months of age in PP and MLN cells. In contrast, IL-3 production of PP, MLN, spleen cells didn't decrease at 21 months of age, but decreased only in spleen cells at 24 months of age. Therefore, it is suggested that the onset and the rate of age-related functional decline of GALT are much later and slower than those of systemic immune system. GALT seems to maintain the immune functions longer than systemic immune system.