Vasoactive intestinal peptide (VIP) and its G protein-coupled receptors, VPAC1R and VPAC2R, are prominent in the immune system and regulate many aspects of T cell-dependent immunity. In mouse T cells, VPAC1R is expressed constitutively, whereas VPAC2R is induced by immune stimuli. VPAC2R-null (VPAC2R-/-) mice on a C57BL/6 background are shown here to have normal basic immune characteristics, including serum Ig concentrations, blood levels of all leukocytes, and spleen number of total T cells (CD3+) and T cells bearing CD4, CD8, and CD28. Hapten-evoked cutaneous delayed-type hypersensitivity (DTH) was significantly enhanced in VPAC2R-null mice compared with age- and sex-matched wild-type mice. In contrast, generation of IgE anti-hapten antibodies and active cutaneous anaphylaxis were ≥70% lower in VPAC2R-null mice than in wild-type controls. Cytokine production by splenic CD4+ T cells, stimulated with adherent anti-CD3 plus anti-CD28 antibodies, revealed higher levels of IL-2 (mean=3-fold) and IFN-γ (mean=3-fold), and lower levels of IL-4 (mean=one-fifth) in VPAC2R-null mice than wild-type controls. Loss of VIP-VPAC2R maintenance of the normal ratio of Th2/Th1 cytokines thus leads to a state of enhanced DTH and depressed immediate-type hypersensitivity, which may alter both host defense and susceptibility to immune-mediated diseases.