The design, control and performance of a versatile regenerative road load simulator for laboratory use is described. The arrangement of the unit provides two separate controllable dynamometers for testing a wide range of conventional, electrical and hybrid propulsion systems and transmission components. A hydrostatic pump-motor set forms one dynamometer; this will absorb or supply a maximum torque of 410 N.m. and up to 70 kW of tailshaft power at speeds from zero to 3000 r/min. The second dynamometer, which can also simulate the behaviour of small to medium sized IC engines, is a Ward Leonard motor-generator set capable of absorbing or supplying up to 150 N.m to 3000 r/min, then 56 kW up to 6000 r/min. Components of this dynamometer can also be set up as a battery simulator to supply or absorb power for electrical components of any hybrid or electric transmission system under test. In this mode the mechanical power is absorbed or supplied by the hydrostatic dynamometer. The two dynamometers can be used concurrently to evaluate such components as axle differentials and epicyclic gear boxes, in which case a separate prime mover is required. Regenerative capabilities in this mode will depend on the power absorbing capacity of the prime mover chosen. The performance of the simulator with a test automatic transmission fitted, when evaluated using the Federal DHEW Urban Driving Schedule velocity-time history, was found to be satisfactory.