We report that cultured rat peritoneal cells spontaneously synthesize nitric oxide and this is associated with active suppression of mast cell secretory function. Addition of interleukin-4 (IL-4) or the nitric oxide synthase inhibitor N-monomethyl-l-arginine to peritoneal cells inhibited nitric oxide synthesis and enhanced anti-IgE-mediated mast cell degranulation, measured as serotonin release. Interferon-γ (IFN-γ) completely overcame the enhancement of serotonin release and suppression of nitrite production induced by IL-4. Over several experiments, with or without IL-4 and/or IFN-γ, serotonin release correlated inversely with nitrite production. On a cell-for-cell basis, non-mast cells produced ≈30 times more nitrite than mast cells in peritoneal cell populations, with or without IFN-γ stimulation. The nitric oxide donor S-nitrosoglutathione inhibited anti-IgE-induced serotonin release from purified mast cells, whereas 8-bromo-cyclic GMP, the guanylate cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one, superoxide dismutase and the peroxynitrite scavenger uric acid, were without effect. We conclude that IL-4 and IFN-γ reciprocally regulate mast cell secretory responsiveness via control of nitric oxide synthesis by accessory cells; the nitric oxide effect on mast cells is direct but does not involve cyclic GMP or peroxynitrite.