We report measurements on the breakdown strength of plastics used in stereo lithography for rapid prototyping. Three epoxy-based photopolymer resins commonly used for stereo lithography were the focus of this work. Test samples were manufactured with an electrically smooth geometry to minimize field enhancements. The thickness of the stressed region in the samples was nominally 1 or 2 mm. Samples were tested to failure by applying a ramped and held voltage pulse at discrete levels up to a maximum of 240 kV. We confirmed the uniform field distribution of the sample geometry with electrostatic modeling and calculated the electric field stress at failure as a simple voltage/thickness ratio. These results are compared with values obtained for several typical materials often used in high voltage applications (acrylic, nylon, etc.). We find that the failure threshold for the SLA materials can be a factor of 2-3 below that of the standard materials.