Tryptophan is transported into the brain by a complex system, experimental and kinetic analysis of which favours a carrier mechanism functioning in association with diffusion (3-paramater model), as distinct from a system comprising two carriers (4-parameter model). The Lineweaver-Burk method is limited as a means of distinguishing between these alternative models. A statistical (linerization) technique represents an improvement over the graphical method in providing the best estimates of the parameters, as well as a measure of the variance (error mean square) of each fit to the data. However, even with this a distinction between alternative models ultimately depends upon analysis of a variety of kinetic experiments.