Studies in human granulosa cells (hGCs), isolated from follicular aspirates of in vitro-fertilisation (IVF) patients, showed that the A-type K⁺ current (I_A) is predominant in these cells. The presence of K_v4.2 and K_v3.4 in follicles and corpora lutea indicates a physiological role in the human ovary of both, the specific channels and I_A itself. The function of this current, which is almost exclusively known in excitable cells, could however not yet be explained in the potentially unexcitable hGCs. In developing follicles specific and changing oxygen and redox-situations exist. While in pig, H₂O₂ appears to be abundant in follicular fluids, in human IVF-derived follicular fluids, we could neither detect H₂O₂ in freshly obtained samples nor immediately after addition of various H₂O₂ concentrations. This points at a highly competent H₂O₂ degrading system in the human follicle and at a possible role for H₂O₂ as part of the cellular microenvironment. Addition of H₂O₂ to cultured hGCs did not raise isoprostan levels, which can be viewed as a measure of lipid-peroxidation and, therefore, cell damage. However, H₂O₂ robustly induced intracellular reactive oxygen species (ROS), which may act as signalling factors. When combined with specific channel blockers Phrixotoxin-2 and BDS-I, H₂O₂ generated ROS-levels were significantly lower, showing that blockage of major I_A components K_v4.2 and K_v3.4 affects redox state sensing or forwarding. These results imply that I_A, which is carried mainly by K_v4.2 and K_v3.4, is a part of a redoxbased signalling system in the human ovary. Supported by DFG KU 1282/5-1