Corporate actions and clinical trial data were some of the catalysts behind the most actively traded stocks on Monday.
Shares of Internet services firm Yahoo moved higher as investor speculated on potential corporate actions. Shares initially moved higher on unconfirmed reports published in an online media newsletter which said CEO Terry Semel could be close to resigning. Yahoo announced Semel's resignation after the close of trading.
CNBC's David Faber reported that Yahoo may be forced to explore strategic alternatives, with potential interested parties including News Corp., AT&T, Time Warner unit AOL, Microsoft and Comcast.
China Medical Technologiesreported solid fourth-quarter earnings on Monday, citing higher sales of diagnostic and tumor therapy systems. The tiny China-based medical device maker also said it expects a sharp rise in 2007 sales.
Graphics chipmaker Nvidia saw shares climb after the company said it bought back 3.3 million shares, according to documents filed with the Securities and Exchange Commission. Nvidia said the stock repurchase was part of its ongoing $1.7 billion buyback plan.
Citadel Broadcasting shares rose after JP Morgan Securities said the radio broadcaster's recent acquisition of ABC Radio Holdings from Walt Disney could boost the stock.
"From formerly being a small-market broadcaster with over 80% of its revenues generated in markets smaller than the top 50, the new Citadel will emerge with vastly greater exposure to the largest markets in the country," said analyst John Blackledge.
Genesco shares gained after smaller rival Finish Line said it would acquire the company for $1.5 billion, or $54.50 a share.
Genesco had recently rejected a sweetened takeover bid of $1.2 billion from Foot Locker.
Merger speculation for aluminum giant Alcoa continued on Monday after a London newspaper said metal mining titan BHP Billiton was mulling a potential $40 million bid for Alcoa.
Among downside movers, Encysive Pharmaceuticals plunged after the U.S. Food and Drug Administration rejected the company's experimental treatment for high blood pressure in the lungs.
The FDA issued a third "approvable letter" for Thelin and said the drug did not demonstrate the effectiveness needed for approval as a treatment for pulmonary arterial hypertension (PAH) and advised the company to conduct additional clinical studies.
"We believe we adequately addressed the issue raised by the FDA in the second approvable letter and we are deeply disappointed in their decision," Encysive said in a prepared statement.
Encysive said the FDA decision may prompt it to make "significant reductions" in its infrastructure and workforce but will discuss plans at a later date.
Peter Kang is a markets writer at CNBC.com and can be reached at firstname.lastname@example.org.