A new technology for printing three-dimensional solid objects from computer-generated images is described. The technology is based on extrusion of a thermoplastic polymer melt as small fibers that bond to form the solid object. This technology is referred to as thermoplastic melt extrusion rapid prototyping (RP). Polymer materials for thermoplastic melt extrusion RP must have the ability to form the structural building blocks of the part. These building blocks are walls, shelves, and bridges. The flow and deformation (rheology) of the melt as it is extruded through the circular die and adheres to the layers already in place govern the process. At this exploratory stage of development, there is no model to specify the extrusion conditions and materials properties relationship to RP performance. Ten marginally acceptable RP materials were identified through extensive RP testing. This study presents the preliminary rheological and thermal analytical characterization of these materials. Characterization of the melt properties is fundamental to the development of a constitutive equation for modeling and to an understanding of the process. The most promising RP materials were found to be crystallizing polymers. Isothermal rheometry and calorimetry measurements were performed to study the crystallization kinetics. Solidification due to crystallization during extrusion is expected to play a key role in RP materials performance.