The relationship between type 1 Fc ϵ-receptor (Fc ϵRI) mediated cell stimulation, Ca 2+-signals and membrane currents was studied in rat mucosal mast cells, subline RBL-2H3 by combining patch-clamp, Fura-2, 45Ca 2+-uptake and secretory response measurements. Cells were stimulated by Fc ϵRI clustering either with IgE and antigen or by an IgE specific monoclonal antibody. Both stimuli induced a biphasic increase in the free intracellular Ca 2+-concentration ([Ca 2+]i). Fc ϵRI clustering in Ca 2+-free solution induces a transient increase in [Ca 2+]i reflecting Ca 2+ release from the Ins(1,4,5)P 3 sensitive stores. Mn 2+ applied to a nominally Ca 2+-free solution, causes quenching of the Fura-2 emission during Fc ϵFRI clustering, indicating activation of a transmembrane pathway for the entry of extracellular calcium ions. Whole-cell current-voltage relationship of resting cells showed strong inward rectification. The inward current component at a potential of −100 mV is increased by 23 ± 11% (n = 14) upon Fc ϵRI clustering, whereas the outward component at +50 mV was enhanced by 45 ± 6%. The Fc ϵRI activated current was identified as due to K + ions, because it reversed close to the K +-equilibrium potential, was blocked by Ba 2+ or Cs + containing or K +-free bath solutions. It was also inhibited by TEA and quinidine, while DIDS had no effect. Moreover, an inwardly rectifying K +-channel with a conductance of 28 pS was recorded in single channel measurements. The open probability of this channel increased by 39 ± 16% (n = 8) upon Fc ϵRI clustering. Superfusion of the cells with nominally K +-free solution also significantly inhibited both the FcϵRI mediated 45Ca 2+ uptake and the secretory response of the cells. We conclude that activation of K +-channels upon FcϵRI clustering is functionally involved in the control and the maintenance of the secretory response of RBL-2H3 mast cells.