NK cell function is regulated by a dual receptor system, which integrates signals from triggering receptors and MHC class I‐specific inhibitory receptors. We show here that the src family kinase Fyn is required for efficient, NK cell‐mediated lysis of target cells, which lack both self‐MHC class I molecules and ligands for NKG2D, an activating NK cell receptor. In contrast, NK cell inhibition by the MHC class I‐specific receptor Ly49A was independent of Fyn, suggesting that Fyn is specifically required for NK cell activation via non‐MHC receptor(s). Compared to wild type, significantly fewer Fyn‐deficient NK cells expressed the inhibitory Ly49A receptor. The presence of a transgenic Ly49A receptor together with its H‐2d ligand strongly reduced the usage of endogenousLy49 receptors in Fyn‐deficient mice. These data suggest a model in which the repertoire of inhibitory Ly49 receptors is formed under the influenced of Fyn‐dependent NK cell activation as well as the respective MHC class I environment. NK cells may acquire Ly49 receptors until they generate sufficient inhibitory signals to balance their activation levels. Such a process would ensure the induction of NK cell self‐tolerance.