Mouse bone marrow-derived mast cells (BMMC), which were differentiated from their bone marrow precursors during 14 days of culture in the presence of a T cell lymphokine, were cultured in the absence or in the presence of 0.01 to 5 microM dexamethasone (DEX) for periods of 6 to 96 hr to assess the effects of steroid treatment on stimulus-dependent mediator release. Preincubation of the BMMC for 24 hr with DEX inhibited the subsequent immunoglobulin (Ig) E-dependent release of beta-hexosaminidase in a dose-dependent manner by up to 38% (mean, n = 7) at 1 microM DEX, and the decrease was statistically significant at DEX concentrations greater than or equal to 0.5 microM. DEX (1 microM) inhibited granule secretion from BMMC in a time-dependent manner; the decrease was statistically significant after a preincubation period of 6 hr, was maximal after preincubation times greater than or equal to 24 hr, and was reversible by 24 hr of incubation in medium lacking the steroid. Preincubation for 24 hr with DEX also inhibited the IgE-dependent biosynthesis and release of the immunoreactive leukotrienes (LT)C4 and LTB4 in a dose-dependent manner by up to 56% (n = 7) and 54% (n = 7), respectively, at 1 microM DEX, and the decrease was statistically significant for each product at DEX concentrations greater than or equal to 0.1 microM. In contrast, BMMC preincubated with DEX for 24 hr before sensitization and antigen challenge demonstrated a statistically significant increase in the release of immunoreactive prostaglandin (PG) D2 at DEX concentrations greater than or equal to 0.1 microM and a maximal enhancement of 188% (n = 3) at 1 microM DEX. The effect of 1 microM DEX on the enhancement of IgE-dependent PGD2 generation was significant after 6 hr, and was maximal by 72 hr. The inhibitory effects of DEX on the antigen-induced release of 5-lipoxygenase pathway products were confirmed by the physical measurements of 5-hydroxy-eicosatetraenoic acid (5-HETE) and LTC4, and by isotopic measurement of [3H]LTC4 after reverse-phase high performance liquid chromatography (RP-HPLC). As assessed by fluorescence cytofluorography of IgE-sensitized BMMC that were interacted with F(ab')2 rabbit anti-mouse epsilon and fluorescein isothiocyanate-labeled F(ab')2 goat anti-rabbit IgG-F(ab')2, the number of IgE-Fc receptors on the cell surface decreased by more than 55% after treatment for 24 hr with 1 microM DEX.+