This paper conducts an empirical investigation of the temporal stability of parameters of freight demand models in the short-medium term using seven national freight origin-destination samples collected in Colombia during the years 1999 to 2005. The paper considers freight generation, freight distribution, and empty trip models. To identify time-dependent effects, models were estimated using a panel formulation with time-dependent parameters and fixed time effects, and compared with the corresponding cross-sectional models. The results indicate the presence of statistically significant time-dependent effects on all freight generation models (production and attraction), freight distribution models (both based on loaded vehicle trips and commodity flows), and empty trips models. The literature review conducted indicates that this article is, if not the first, one of the few publications studying the temporal stability of parameters. The results show a remarkably consistent pattern in that the components of freight demand that could change faster (i.e., freight production and attraction) are the ones that exhibit the largest rates of parameter change. As shown, the rates of change for these models are 18.29% and 26.37% respectively. In contrast, the freight distribution models of loaded trips were found to change less rapidly (10.50% and 1.94% depending on the impedance function); while the tonnage distribution model only exhibited fixed time effects. The model that changes least rapidly is the empty trip model, with a rate of change of 0.83%.