Wall Street underwriters kept busy in the second quarter, yet ebbing investor appetite for lower-rated debt and riskier assets may dampen their summer.

In the last couple of weeks, investors have grown more wary of risk, amid struggles at two Bear Stearns Cos. hedge funds loaded with illiquid mortgage debt. Several companies restructured junk bond offerings. Corporate raider Carl Icahn said the private equity boom has peaked. And while Blackstone Group LP conducted a $4 billion initial public offering, its shares fell below the IPO price less than a week later.

These are bad signs for investment banks due to push 10- and 11-figure bond offers to fund leveraged buyouts (LBOs) such as payment processor First Data Corp. and power company TXU Corp. Home Depot Inc. will try to convince bond investors to absorb $12 billion of debt as it sacrifices its very high credit ratings to fund a giant stock buyback.

We're in a one-hand, other-hand type of environment, said Therese Esperdy, JPMorgan Chase & Co.'s head of global debt capital markets. The economy seems solid now, but people question whether we are at the top of the cycle. Investors remain concerned about potential investment-grade credits disappearing into LBO land, and shareholder activism finally taking root in the number and size of leveraged share repurchases. Citigroup Inc. kept its crown as Wall Street's top underwriter of stocks and bonds in the first half, while Merrill Lynch & Co. (displaced it as the top underwriter by reported fees, Thomson Financial said on Friday.

The volume of securities offerings totaled $4.53 trillion, up 15 percent from a year earlier. Citigroup, JPMorgan and Deutsche Bank AG retained their top three rankings, handling a respective $398.1 billion, $338.5 billion and $291.2 billion of volume.

Disclosed fees edged up 2 percent to $7.7 billion. Merrill took in $869.7 million, followed by JPMorgan's $768.3 million and Citigroup's $654.1 million. Citigroup had ranked first a year earlier, while Merrill ranked fourth and JPMorgan fifth. Investment bankers compete intensely for high rankings from Thomson for both pride and marketing reasons.