Genzyme rejected Sanofi-Aventis' $69 per share offer today, calling it "unrealistic." Shares of Genzyme closed at $69.91 per share, as investors see more value in the company and hope for a higher offer. Genzyme's stock had traded above the $80 range back in 2008 before the recession. Management believes the share price can head back there, thus any subsequent offer would have to be much higher.
Similarily, Potash Corp received an offer of $130 per share which was rejected imediately. The stock quickly rose to $147 in anticipation of a better offer.