Synchronization of a certain type of valve oscillator, having a low-pass filter in the synchronization loop, and also of self-starting synchronous motors is examined. Under some conditions the pulling effect takes place in these circuits, i.e. the pull-in range of synchronization is smaller than the pull-out range. The pulling function, i.e. the relation between parameters of the circuit and the ratio of the pull-in and the pull-out range, has previously been examined only qualitatively in relation to synchronous motors. The quantitative examination presented here yields simple formulae applicable to design problems. An asymptotic approximation is also found to the pulling function in a synchronized system possessing a limitation in the synchronization loop