Shares of Swiss farm chemicals power Syngenta leaped more than 14 percent in post-market trade Thursday after a report that US seed giant Monsanto had made a takeover offer.
Citing two people familiar with the matter, Bloomberg News reported the two global agroindustry leaders held discussions in recent weeks.
But it said Syngenta "has concerns" about a merger, because it could be challenged by regulators on antitrust grounds.
The combined company would be the world market leader in both seeds and crop chemicals.
It was not the first time talks between the two have been reported. Rumors of a possible merger surfaced in June 2014, but nothing came of them.
No prospective price was given in the report, but Syngenta has a market value of about $31 billion, while Monsanto is worth $54 billion.
Neither company could be immediately contacted on the report.
Syngenta's New York Stock Exchange-traded ADR shares surged 14.1 percent in post-market trade to $76.50 after having fallen modestly in regular trade.
Monsanto meanwhile was 3.6 percent higher at $118.07.