Bob Carter, who heads the Toyota brand in the United States, said it was more important for Toyota to understand consumer expectations and hone the battery-centered technology behind plug-in cars than to race to bring them to showrooms.

"Before we bring it to market, our customers always expect a level of quality and reliability, value and cost," Carter said. "It's critical that we understand the expectations of the consumers."

Environmental advocates, especially in California, have been pressing automakers to roll out plug-in vehicles that would be capable of recharging at standard outlets with a traditional engine to provide power for longer-haul driving.

GM has set a target date of 2010 for its first plug-in hybrid, the Chevrolet Volt.

Toyota, which dominates the current market for hybrids, has not discussed its timetable for plug-in vehicles. Toyota is expected to account for about 80 percent of all hybrids sold in the United States this year, led by the Prius.

Toyota said on Friday it would deliver one modified Prius each to the University of California, Berkeley and the University of California, Irvine for a three-year study meant to speed up development of plug-in versions of the hybrid.

Researchers working with Toyota at Berkeley will concentrate on consumer behavior, sounding out their view of plug-in hybrids before and after driving them.

"We will be looking at lifestyles -- how people are using the vehicles," said Susan Shaheen, research director of the Transportation Sustainability Research Center at UC Berkeley. "Where they drive, how they recharge the vehicles, when they recharge the vehicles. And we will ask them a great deal about their perceptions."

Researchers will seek to learn how long drivers want the plug-ins to run on electric power only, how much they will pay for one, and where they will use charging stations. Shaheen said the first results of the Berkeley consumer studies would be ready in about a year.

UC Irvine will concentrate on technical issues, such as how much electricity will be taken from the regional power grid as thousands of hybrid owners plug in.

"Using real-world settings, we will begin to answer some of these open questions," said Scott Samuelsen, director of the Advance Power and Energy Program at UC Irvine.

Carter said Toyota would make "significant announcements" at next week's Los Angeles Auto Show and Detroit's auto show in January regarding advances to vehicles that feature alternatives to traditional combustion engines.

He said Toyota has no intention of leasing more expensive batteries for next-generation hybrids separate from the vehicle, a step some proponents have suggested as a way to bring down the initial cost of such vehicles.

Toyota is working now to develop the third-generation of the Prius hybrid, expected to go on sale in the United States in the 2010 or 2011 model years.