It has been the best of times, as Schwarzman noted in his January keynote address at the Wharton Private Equity and Venture Capital Conference. But, he added, nothing lasts forever.

"Nobody knows when or why it"—the end of easy and cheap credit—"will happen," he said. "But it's hard to imagine it can get better than it is. We're at maximum advantage, in all probability, right now."

However, these driving forces are already showing signs of fatigue. Competition from other private equity firms and hedge funds is increasingly pushing up the cost of acquisitions. (While Schwarzman may not have really wanted to pay $39 billion to beat out rivals for Equity Office Properties Trust, as he told the Wall Street Journal, "I always think about what will kill off the other bidder.")

The stock market is giving a better valuation to many possible targets, in part because of takeover fever. Long-term-bond yields have risen, which has in turn increased the cost of borrowing. And some lenders are tightening their purse strings, or at least thinking about it.

Blackstone has confined its comments to filings with the Securities and Exchange Commission, citing rules forbidding securities issuers from speaking publicly immediately before an I.P.O. "We are gagged and bound by the S.E.C. at the moment unfortunately," Blackstone spokesman John Ford said.

In those filings, Blackstone states that it believes "the long-term growth trends in our businesses are favorable" but adds that there "may be significant fluctuations in our financial results from quarter to quarter" and that investors should expect to "remain unit-holders for a number of years."

The filings notes several risks, including possible jeopardy to the tax treatment underpinning the endeavor. "Current law may change" and "the value of our common units would be adversely affected," one filing says.