An improved air distribution system for cooling electronic components mounted on printed circuit boards. The system includes an air plenum chamber of rectangular cross section. Openings are formed in the side walls of the chamber to permit air under pressure from within the chamber to flow outwardly in a direction substantially normal to the outer surfaces of the side walls and over the components to be cooled. A baffle in the form of a thin sheet of metal is mounted in the plenum substantially parallel to the side walls to divide the interior into two substantially equal portions. A large number of small round holes are formed through the baffle the area of the holes occupying substantially 36 percent of the total area of the baffle. An air pump supplies air under pressure to the interior of the plenum chamber. The baffle modifies the pressure distribution and air flow within the chamber so that the velocity of air through the openings formed in the side walls does not substantially vary with the position of the openings along the length of the plenum.
