1938: GM's Electro-motive Corporation began supplanting steam locomotives with diesel-electric hybrids. A large IC engine driving a generator supplies power to electric motors which drive the engine's wheels. No electrical energy is stored on-board.

1939: Electrogear buses manufactured and operated in Philadelphia used a differential transmission capable of blending IC and electric power for heightened efficiency.

1944: P. M. Heldt's Torque Converters or Transmissions for Use with Combustion Engines in Road and Rail Vehicles, Tractors and Locomotives was published. This handy reference soon became the preferred source of inspiration for hybrid engineers the world over.

1969: GM engineers in England constructed a two-seat minicar with a parallel-hybrid drive, a 35 mph top speed, and a 150-mile range.

1970: The Clean Air Act renewed interest in hybrid propulsion. Four TRW engineers working under Department of Health, Education, and Welfare (EPA precursor) auspices invented a clever electromechanical transmission for hybrid vehicles.

1973: The first energy crisis intensified the search for more fuel efficient transportation.

1974: Dr. Victor Wouk and Dr. Charles Rosen installed what they termed a compound parallel hybrid propulsion system in a Buick Skylark. The five-passenger, 4100-pound sedan demonstrated a 200-mile range, 80-mph top speed, and reductions in fuel consumption and emissions.

1976: After Congress passed the Electric and Hybrid Vehicle Research, Development, and Demonstration Act, General Electric was tapped to construct a parallel-hybrid sedan capable of doubling fuel efficiency.

Toyota built its first hybrid, a small sports car with a gas-turbine generator supplying current to an electric motor.

1980: Briggs & Stratton sponsored the construction of a six-wheel compact with parallel-hybrid propulsion. The 26-hp (combined), 3200-lb multi-mode vehicle had a 75-mph top speed.

1989: Audi presented the Duo experimental vehicle with an IC engine powering the front wheels and electric rear drive.

1993: The Clinton-Gore administration created a Partnership for a New Generation of Vehicles (PNGV) between the United States Council for Automotive Research (formed in 1992), and a network of universities, national labs, federal agencies, and suppliers. The goals were 80-mpg concept vehicles by 1999 followed by production-feasible prototypes by 2004. No prototypes emerged, though GM's Precept did achieve 90 mpg on diesel fuel.

Toyota's exclusion from PNGV moved chairman Eiji Toyoda to ponder more efficient automobiles. Takeshi Uchiyamada was assigned the chief engineer's job for a project called G21 (global car for the 21st century).

1994: The original goal of a 50-percent efficiency gain was doubled by Toyota's new engineering executive vice president Akihiro Wada who targeted the following year's Tokyo Motor Show as the ideal opportunity for displaying a hybrid concept.

1995: While the Toyota Prius concept was under construction, eighty research engineers brainstormed on a practical hybrid powertrain. The final Toyota Hybrid System (THS) selected in June combined one IC engine, two electric motor-generators, and a planetary gear set in a configuration identical to TRW's 1970 electromechanical transmission. The first THS prototype ran in December.

1996: The Prius's market introduction was accelerated two years so Japanese customers would be on the road before the Kyoto Conference on Global Warming held in December 1997.