Two of the world's leading exchanges, Deutsche Boerse and NYSE Euronext, are in advanced talks about a merger.
The deal would create the world's biggest stock exchange firm by revenue and profits and would be jointly based in New York and Frankfurt.
But the two warned there was no guarantee that discussions would end with a deal.
The announcement came on the day that the London Stock Exchange (LSE) confirmed a tie-up with Canada's TMX.
"This transaction creates a group that is both a world leader in derivatives and risk management and the premier global venue for capital raising," a joint statement from Deutsche Boerse and NYSE Euronext said.
Deutsche Boerse shareholders would own between 59%-60% of the combined exchange, while NYSE shareholders would hold 40%-41%, it said.
The two would be able to save about 300m euros ($411m; £255m) through merging, as well as boosting revenues, it added.
Deutsche Boerse boss Reto Francioni would become chairman, while his counterpart at NYSE, Duncan Niederauer, would take the role of chief executive at the new group.
"That's as big as deals get," said Ed Ditmire at Macquarie Securities. "Clearly Deutsche Boerse would be the buyer. It would be a stretch to call it a merger of equals."
If the deal goes ahead, it would overshadow Wednesday's merger of the LSE and TMX, which operates the Toronto Stock Exchange.
This merger is the latest in a long line of deals between leading stock exchanges around the world.
In recent years, the New York Stock Exchange has bought Euronext; Deutsche Borse has taken over the International Securities Exchange in the US; and the London Stock Exchange, which has successfully fought off a number of takeover approaches, has bought Borsa Italiana.
Analysts say they expect further consolidation among other exchanges.