Some environmental groups have pushed for plug-in hybrids, called PHEVs, or plug-in hybrid electric vehicles, as a way to save on gasoline, thus curbing emissions.

But some experts say plug-ins may not be the ultimate answer to cutting pollution, if the electricity used to charge them comes from coal-fired power plants.

That is also a concern to Toyota, which has asked researchers to determine not only whether consumers would be willing to pay for a plug-in, but also the effect it would have on the environment, James Lentz, the president of Toyota Motor Sales, said in an interview Sunday.

Nonetheless, G.M., Toyota and Ford Motor, the world’s three biggest car companies, all are developing plug-in hybrid vehicles. Along with the Volt, G.M. has said it plans to produce a plug-in version of its Saturn Vue hybrid. Ford has not yet given details of its plug-in hybrid, which it first discussed in 2006.

Indeed, Toyota executives initially questioned the practicality of plug-in hybrids, saying consumers preferred the convenience of hybrids that did not have to be recharged. Toyota has sold more than one million hybrids worldwide, including more than 800,000 Prius cars.

But the automaker announced last July that it was testing plug-in hybrids on public roads in Japan. It also is testing them in France, Toyota officials said Sunday, and it has given prototype versions of plug-in hybrid vehicles to university researchers in California.