Shares climbed by 2.34 percent to 406.20 francs in morning trade in Zurich.
They had bolted 19.3 percent on Friday after the Basel-based firm turned down the offer.
Syngenta said Monsanto had made an offer of 449 Swiss francs ($486) per share with approximately 45 percent in cash, but claimed it was undervalued.
The US firm had timed the offer with a slackening in Syngenta's performance but the Swiss firm said the proposal still did not "reflect the outstanding growth prospects" of the company.
"The offer fundamentally undervalues Syngenta's prospects and underestimates the significant execution risks, including regulatory and public scrutiny at multiple levels in many countries," it said.
Syngenta chairman Michel Demare said although the company's "valuation is currently affected by short term currency and commodity price movements, the business outlook is strong, with emerging markets accounting for over 50 percent of our sales."
Some analysts said on Monday that Monsanto could make an improved offer.
Markus Mayer, an analyst at Baader Helvea said it was " just the beginning in the takeover game" and that the US company could make an offer of 500 Swiss francs per share.
Patrick Rafaisz, an analyst at Vontobel, also said that several scenarios could be considered, including Monsanto pushing with an unfriendly bid or
offering a higher price. He also forecast a price of around 500 francs saying "this story is not yet over."
The companies held preliminary talks last year with advisers about a combination, before Syngenta's management decided against negotiations,
sources told Bloomberg at that time.
Syngenta was formed in 2000 by the merger of Novartis Agribusiness and Zeneca Agrochemicals.