The invention relates to an inhalator (1) for powdery substances (33), especially medical substances. The inhalator comprises a substance reservoir chamber (27) and a metering chamber (34). The metering chamber is configured as a crossbore (35) of a rod (36) and can be displaced from a filling position (B) to a discharge position (E), in which discharge position (E) the metering chamber (34) is positioned in an air flow channel (21). The aim of the invention is to improve said inhalator, especially with regard to the filling or emptying of its metering chamber, even when particles of smallest dust-like grain size are used. For this purpose, at least a part of the reservoir chamber wall (28) is displaced in the direction of the rod (36) when the inhalator (1) is actuated.