In a combined mechanical and electronic key, electronic components and mechanical flat keys (30) are normally housed in a common key holder (10). In order to place the flat key (30) between a lowered rest position in a holder (10) and a projecting in-use position, the flat key (30) is movably located in a container (10) and secured in at least one of the positions by a push button (40). The key contain is assembled from an upper and a lower shell. In order to avoid sealing problems between both shells, according to the invention, the upper shell (11) is provided with an outbreak in a region pertaining thereto which lies outwith the push button. The outbreak creates a void chamber which can be accessed from the outside and is located on the inside of the shell interior. The electronic components are enclosed by a housing-like capsule and form therewith a prefabricated electrocapsule (20). The electrocapsule (20) forms a socket unit, which can be inserted thereafter in the void chamber pertaining to the pre-assembled key container (10). The electrocapsules (10) are secured in the key container (10) when inserted in the socket. The push button (40) is used to advantage for securing.