Carlos Ghosn, CEO of both Renault and Nissan Motor, faced some petulant shareholders at the French automaker's annual meeting on Apr. 29 in Paris. One complained that the stock was down more than 30% this year. What, he wanted to know, had happened to the Ghosn Effect?

Good question. The Ghosn Effect once referred to the galvanizing impact the CEO had on Nissan, when he was dispatched by Renault, Nissan's controlling shareholder, to turn around the foundering Japanese company almost a decade ago.

Today, Ghosn's critics use the phrase to describe a boss who they worry may be stretched so thin he cannot manage either Nissan or Renault as well as he could manage one company. He certainly has hit a boggy patch. Sales at Renault have flattened out. Profits last year slid 7.6% and won't do any better in 2008. And Nissan? Its stock is down 32% from its 12-month high on flat profits in 2007 and poor prospects for 2008.

Yet Ghosn, a.k.a. Le Cost Killer, Super Carlos, the Icebreaker, and probably a dozen other epithets, isn't ruffled by the critics. True, his schedule seems impossible for ordinary mortals to survive, let alone thrive on. On Saturday, Apr. 26, he was in Tokyo preparing for Nissan's May 13 annual meeting and announcement of a new five-year plan. Monday night it was New York to pick up an award (after lunch with BusinessWeek). Tuesday afternoon he was in Paris to meet those agitated shareholders.

And, says Ghosn, it's all worth it. If you look at car-company tie-ups, such as Daimler-Benz buying Chrysler or Ford Motor (F) buying Jaguar and Land Rover, "there is only one global alliance that has added value, and that is Nissan and Renault," he says, pointing out that the two are still earning billions even in a year of recession and slowdown. Renault's operating income hit $1.8 billion in 2007, while Nissan posted $3.1 billion in operating income for the first half of its current fiscal year.

Ghosn figures his dual-CEO role fits the times, when automakers must move fast to adapt to changing tastes and ever-tougher regulations. In 2007 he launched four major joint ventures involving Nissan and Renault: two electric car projects in Denmark and Israel, a combined factory in Morocco, and a project to design and build a $3,000 car in India. That's on top of winning a joint venture in Russia with AvtoVAZ, securing a deal to supply small cars to Chrysler, outsourcing Nissan's pickup-truck manufacturing in the U.S., and agreeing to build small pickups for Suzuki Motor. "You need one decision-maker for these things," says Ghosn, "so you get the bickering [that would result from having two CEOs] off the table."

The game for Ghosn is getting even more challenging than when he saved Nissan from bankruptcy. In his worldview, Nissan has to stay strong in the U.S. even though, he says, "the U.S. market is not going to be great again." A saturated America now has some 800 vehicles per 1,000 people, vs. fewer than 30 in China and India. Hence the dilemma: You have to stand your ground in the huge U.S. marketplace while racing to win the hearts and pocketbooks of first-time car buyers in China, Brazil, Russia, and elsewhere.