New stock was issued at a breakneck pace in the first half of 2006, as companies tapped global markets both to expand operations and take over their peers. But the picture might not be as robust as the year progresses.

In the first half of 2006, the amount of stock sold in initial public offerings rose 56%, compared with the same period last year, to $102 billion, according to preliminary data from Dealogic. Total equity issuance, which includes stock used to fund takeovers and sold by companies that already were public, rose 46% to $360.6 billion. In the U.S., the volume of stock issued rose 67%.

The rate of growth in Asia was higher than in both Europe and the U.S. For example, equity issuance in South Korea rose 161% in the first half of the year, the biggest jump of any country. Japanese companies issued equity at twice the pace of last year. In IPOs, Italy and France topped the list, with deals in France up 193%, while deals in Italy rose 144%.

The dollar value of all mergers and acquisitions rose 48% in the first half of the year to $1.8 trillion, according to preliminary data from Thomson Financial, compared with the same period last year.

After much buzz about the expanding role of boutique investment banks in providing mergers advice, none ranked among the top 10 advisers for the first half of the year. Goldman Sachs ( GS) remained in the top spot, advising 206 mergers and acquisitions worldwide worth over $655 billion. Citigroup ( C) trails Goldman, with 176 deals worth $532 billion, and JPMorgan Chase ( JPM) is third, with 209 mergers worth $486 billion. Among the boutiques, Lazard ( LAZ) is ranked 11th and Rothschild 12th.

Meanwhile, many of the deals in the second quarter came in the last two weeks. The financing of those deals will start early in the third quarter, when the environment in stock and bond markets is anything but certain.

Two multibillion-dollar deals announced recently are prime examples. Anadarko ( APC) announced a $23.3 billion deal to acquire two competitors last Friday. It plans on issuing debt and equity later in the year to finance the deal. But many debt strategists are concerned about how the financing will proceed , given the uncertain interest rate environment.

Univision ( UVN) also could also have problems financing its deal. The company recently sold itself to a group of private equity investors for $12.3 billion. Because the Spanish broadcasting firm operates radio and television stations in an attractive corner of the media market, private equity investors have been aggressive with the debt they will issue to close the deal. Lenders will be asked for more than $8 billion to finance the transaction.

These aggressive transactions will be accompanied by a slew of other large deals with equally hefty financing packages. All the companies will ask investors for financing all at the same time. As they compete for investor dollars in a touchy market, some mergers could struggle.