Abstract: A driver's seat for tractors or the like has a system such as crossing levers or a parallelogram for keeping it horizontal on changes in level, which take place when a spring under the seat is compressed. For making possible adjustment to different weights of drivers without changing the natural frequency of the springing motion upwards and downwards of the seat, the spring has two runners acting on cams on the levers of the system so that, when the spring is moved in relation to the cams, the leverage of the spring by way of the levers on the top part of the seat is changed. Such horizontal motion of the spring is produced by a carriage pushed backwards and forwards by a threaded rod with a handwheel.