A regulation assembly for a respiratory mask for an aircraft crew member, said assembly comprising: - a support , - a mode selection button capable of being moved between at least a first position (EMER), a second position (100%) and a third position (N), the second position (100%) being positioned between the first position (EMER) and the third position (N), - a regulator intended to be supplied by a source of respiratory gas and adapted to supply a respiratory cavity at least in three following operating modes: - when the mode selection button is in the first position (EMER), the regulator supplies the respiratory cavity as long as the pressure in the respiratory cavity is not greater than a first relative pressure with respect to the ambient pressure, - when the mode selection button is in the second position (100%), the regulator supplies the respiratory cavity as long as the pressure in the respiratory cavity is not greater than a second relative pressure with respect to the ambient pressure, the first pressure being greater than the second pressure, - when the mode selection button is in the third position (N), the regulator supplies the respiratory cavity with respiratory gas diluted with air, characterised in that the mode selection button includes a base pivotally mounted on the support relative to an axis of rotation (Z), and a cover carried by the base ; said axis of rotation (Z) being perpendicular to the support and extending in an axial direction (A); said cover being movable in the axial direction (A) relative to the base between a locking position of the mode selection button and an unlocking position of the mode selection button. - a support , - a mode selection button capable of being moved between at least a first position (EMER), a second position (100%) and a third position (N), the second position (100%) being positioned between the first position (EMER) and the third position (N), - a regulator intended to be supplied by a source of respiratory gas and adapted to supply a respiratory cavity at least in three following operating modes: - when the mode selection button is in the first position (EMER), the regulator supplies the respiratory cavity as long as the pressure in the respiratory cavity is not greater than a first relative pressure with respect to the ambient pressure, - when the mode selection button is in the second position (100%), the regulator supplies the respiratory cavity as long as the pressure in the respiratory cavity is not greater than a second relative pressure with respect to the ambient pressure, the first pressure being greater than the second pressure, - when the mode selection button is in the third position (N), the regulator supplies the respiratory cavity with respiratory gas diluted with air, characterised in that the mode selection button includes a base pivotally mounted on the support relative to an axis of rotation (Z), and a cover carried by the base ; said axis of rotation (Z) being perpendicular to the support and extending in an axial direction (A); said cover being movable in the axial direction (A) relative to the base between a locking position of the mode selection button and an unlocking position of the mode selection button. <EOS>