The present invention is directed to an automatic reconstitution drug delivery device. The inventive device effects simple and automatic reconstitution of a dry drug. To effect reconstitution, a user displaces a release button which causes first and second chambers of a drug cartridge to communicate, resulting in automatic mixing of a diluent and dry drug to effect reconstitution of the dry drug. The user then simply sets the dose volume using a dose-setting mechanism, and administers the injection in a manner typical of self-injection drug delivery devices. Whole or partial automatic priming can be also achieved by the device.