We show that the assumption of 5D null particle trajectories in five-dimensional Kaluza-Klein gravity can correspond to either four-dimensional massive or null trajectories when the parameterization is chosen properly. We retain the extra-coordinate dependence in the metric and show the possibility of a cosmological variation in the rest masses of particles, and a consequent departure from 4D geodesic motion by a fifth force. ©1999 American Institute of Physics.