Disclosed is a dumbbell of the type commonly found in a fitness center to be used as a weight training device during a weight training or exercise program. The dumbbell includes a gripping handle and a pair of collars fixedly connected to opposites ends of the handle. A series of weight plates having correspondingly different weights are adapted to be detachably connected to each other and to each one of the collars so as to enable a user to selectively and incrementally adjust the gross weight of the dumbbell to conform to his weight training program. Each weight plate carries a lock cartridge having an engagement pin moving therethrough between retracted and extended positions in response to a manual pushing force. In the extended position, the engagement pin of an additional weight plate is received within an engagement pin extension channel of an existing weight plate being carried by the dumbbell, whereby to prevent a disconnection of the additional weight plate from the existing weight plate.