Monsanto, the world's largest seed company, said it still believes in the value of a combination, but will focus on building its core business and meeting long-term growth objectives.
Monsanto confirmed that it made a revised offer to Syngenta on Aug. 18, raising a previous offer to 470 Swiss francs per share, valuing the company at roughly $47 billion. It also confirmed it raised a reverse break-up fee offer to $3 billion.
But, Monsanto said in a statement, Syngenta continued to spurn the overtures.
"Without a basis for constructive engagement from Syngenta, Monsanto will continue to focus on its growth opportunities built on its existing core business..." the company said in a statement.
There was no immediate comment from Syngenta.
A source close to Monsanto said top officials from both companies met in person and talked over the phone, the last such communication being held last week when the offer was raised.
Syngenta told Monsanto Wednesday morning that it was rejecting the offer, the source said.
Monsanto has said that it wanted to acquire Syngenta primarily to boost its agrichemicals portfolio, which now relies mainly on glyphosate-based herbicides branded as Roundup.
Monsanto is known for its development of genetically altered corn, soybeans, canola and other crops, while Syngenta is the world's largest agrichemical company.
The takeover effort became a public spectacle of sorts over recent months as leaders at both companies argued the merits of Monsanto's proposed combination in an unusually public way, taking to online video postings, and other means to debate the deal.
Syngenta's management team insisted Monsanto was undervaluing the company and that an attempted combination would raise serious antitrust issues in many countries, possibly provoking lengthy and costly delays to any merger attempt.
Monsanto, meanwhile, insisted that it could handle antitrust hurdles, and said it would sell off Syngenta's seeds and genetic traits businesses.
Syngenta shares fell more than 18 percent to 309.50 Swiss francs, while Monsanto shares jumped more than 7 percent to $96.04. Monsanto's share boost was helped by news that with the Syngenta pursuit set aside, Monsanto will resume a share buyback program.