Abstract: An operating mechanism for the window in a motorcar door and the like includes guiding devices, including at least one deflecting element, which engage a wire, cable or other elongated tension member and guide the tension member on the door in a loop-shaped path when the tension member is driven in the path alternatively in two opposite, longitudinal directions. The motion of the tension member is transmitted to the window. The tension member stretches under the longitudinal stresses exerted thereon by the drive and the window and needs to be tensioned. According to the invention, a spring in a permanently stressed condition biases the deflecting element in a direction to lengthen the path of the tension member and for thereby compensating for slack in the engaged portion of the tension member when the tension member moves in one longitudinal direction, whereby the spring reaches a partly relaxed condition.