An investigation on a two-dimensional wing model with a double-slotted trailing edge flap is described where tunnel wall boundeo'y layer control by blowing was applied to prevent premature flow separations at the junctions between the model and the tunnel walls. From the results it is concluded that tunnel wall boundary layer control at the wall junctions is necessary to obtain useful results from two-dimensional high-lift tests in windtunnels. It is shown that a relatively simple system of compressed air blowing slots in the tunnel walls gives a sufficient approximation of the desired two-dimensional flow pattern. The blowing system has already been applied on a routine basis to wing sections with trailing edge and also leading edge high-lift devices.