Because they are easy to create and modify, low fidelity prototypes are commonly used in early evaluations of user interface designs. Designers typically use either pen-and-paper or various computer-based tools to create and test low fidelity prototypes. However, our informal analyses of these technologies indicate that they do not optimally support the two key, complementary tasks of (a) prototype creation and (b) wizard-of-oz testing. To address this problem, we have developed WOZ PRO, a pen-based software environment for the quick and easy creation and testing of low fidelity user interface prototypes. When designing interface screens in WOZ PRO, designers can easily (a) propagate a design change to other related screens, and (b) specify the set of screens that are reachable from a given screen. In a wizard-of-oz test, WOZ PRO aims to reduce the cognitive load on the wizard by allowing navigation only to those next screens that are valid. An experimental comparison of WOZ PRO and pen-and-paper provides evidence of WOZ PROpsilas potential, and insight into how its design might be improved.