Planetary gearboxes are used in a large variety of mechanical systems. Several of these systems undergo variable operating conditions, producing non-stationary features of measured variables typically used for condition monitoring purposes, such as vibrations. Benefits can be obtained from these signals only if the consideration of the non-stationary features is properly made. For this purpose it is necessary to conduct numerical investigation and experimental study on the dynamic behaviour of planetary gearboxes under variable operating conditions. In this paper we address the problem of a one-stage planetary gearbox driven by an asynchronous motor in open-loop control configuration and subjected to periodically variable load. The dynamic response is obtained by integrating the equation of motion using implicit Newmark algorithm. For validation, test bench measurements are performed on a planetary gearbox subjected to the same conditions simulated. The results show good agreement between the model and actual system.