The building of population pharmacokinetic/pharmacodynamic (PK/PD) models is a time consuming and complicated task. This is partly due the lack of specialized tools for the visualization and exploration requirements of this type of analysis. In this paper we present Xpose, a model building aid for population PK/PD analysis using NONMEM, which simplifies the task of producing documentation, data set checkout plots, goodness of fit plots and graphical model comparison. It also facilitates covariate model building by the use of stepwise generalized additive modeling (GAM), bootstrap of the GAM analyses and tree based modeling. The plots and analyses are presented in the form of a text based menu system and the only thing the user has to do is to make NONMEM produce one or more table files named in a specific way.