Corporate takeovers and earnings news were some of the catalysts behind the most actively traded stocks on Monday.
Major pharmaceutical company Merck reported quarterly earnings above Wall Street estimates, sendings shares up more than 5%. The Whitehouse Station, N.J.-based company reported non-GAAP earnings of 82 cents a share, compared with analysts' consensus forecast of 72 cents.
Shares of oilfield services firm GlobalSantaFe rose after it agreed to be acquired by larger rival Transocean for almost $18 billion.
Halliburton also gained after the company posted strong quarterly earnings on Monday. The stock hit a new 52-week high after the world's second-largest oil services company said second-quarter profit rose 19%, above analysts' estimates.
Tech hardware giant Hewlett-Packard said it would acquire business software makerOpsware for $1.6 billion, sending shares of the latter zooming up 35%.
Both Ev3 and FoxHollow Technologiessaw shares rise after the medical device makers agreed to merge in a transaction worth about $780 million in cash and stock. Bear Stearns analyst Rick Wise said in a note to clients that the deal is "highly complementary" and maintained an "outperform" rating on shares of Ev3.
"The EVVV-FOXH merger ... seems to be a perfect complementary fit with each salesforce marketing to the same doctors--interventional radiologists," the analyst said.
"The combined salesforce of approximately 240 reps will be one of the largest in the endovascular space at nearly twice the size of other players like Johnson & Johnson, Boston Scientific, Abbott Laboratories and Medtronic."
Shares of Tellabs gained on speculation Nokia Siemens Networks, a joint venture of Nokia and Siemens ,may be mulling a takeover offer for the telecom equipment maker, according to a report from TheStreet.com. A Tellabs spokesperson said the company does not discuss rumors or strategic plans for competitive reasons.
Peter Kang is a markets writer at CNBC.com and can be reached at email@example.com.