Despite recent advances in the cellular and molecular analysis of induction and regulation of mucosal immune responses, little is yet known about differences which occur in aging. To address this important issue, we have compared the mucosal and systemic immune responses of aged (12- to 14-mo- or 2-year-old) and young adult (6- to 8-wk-old) C57BL/6 mice. Both aged and young mice were immunized weekly with three oral doses of 1 mg of OVA and 10 μg of cholera toxin (CT) as mucosal adjuvant. Both groups of mice over 1 or 2 years of age showed reduced levels of Ag-specific mucosal or systemic immune responses at day 21. An Ag-specific B cell enzyme-linked immunospot assay confirmed these results at the cellular level. When the Ag-induced cytokine responses were examined at both protein and mRNA levels, CD4+ T cells from spleen and Peyer's patches of young adult mice revealed elevated levels of IL-4 production; however, these cytokine responses were significantly diminished in aged mice. In contrast to mucosal immunization, mice s.c. immunized with OVA plus CT resulted in impaired OVA-specific but intact CT B subunit-specific immune responses in 12- to 14-mo-old mice although the responses to both Ags were depressed in 2-year-old mice. These results provide the first evidence that the development of age-associated alterations possibly occurs earlier in the mucosal immune system than in the systemic immune compartment.