Abstract A new concept, based on the use of random signals, is proposed for a computing instrument for simulating the reliability aspects of a system. Random signals interact with deterministic signals. A simulator has been designed and constructed using large-scale integrated circuits. A high degree of parallelism has been incorporated in the design, and the use of asynchronous time-scaling techniques have yielded operational speeds of the order of 10 8 computations per second. Experiments have been carried out on unreliable systems using the simulator and typically, system lifetimes of 10 7 hours are simulated by the equipment in approximately on second.