A gas exchange control mechanism for an opposed-piston engine which opens and closes inlet and outlet slots provided in the cylinder irrespective of the position of the pistons. The opposed pistons are guided completely or partially during stroke in sliding sleeves during operation of the engine. The sleeves may reciprocate mechanically, electrically, pneumatically or hydraulically in a linear manner. The sliding sleeves are adapted to open and close gas guide channels located in the engine housing.