For testing a logic unit under test (UUT), rule-based random irritation of a UUT model is provided to be used in conjunction with a simulator. The UUT model is stimulated (or irritated) with data patterns randomly generated by a pattern generator within the boundary of limitations imposed by a rules list. The rules list provides restrictions or encouragements on how data patterns are to be applied to the software model of the UUT. The pattern generator may be implemented either within or outside the simulator. If the pattern generator is incorporated into the simulator, then a software environment is required to interface communications between the pattern generator, the simulator, and other software entities involved in the simulation.